The problem is no one is scripturally required to obey unscriptural rules.
Granted. But how is that relevant? Are you suggesting that a "Reserved for Clergy" sign is an unscriptural rule?
A lot of assumptions are being made, many of them having no basis in fact, so I encourage you to reconsider the dissent.
Well, for one thing, several of those assumptions you listed were not involved in my post or my thinking (e.g., "total disregard for the pastor"). So, "a lot of assumptions were being made" but not by me.
For another thing, I was making a lot more inferences than assumptions. For example, you said that you were there to buy books, that it would take about 15 minutes, and that you were willing to take the chance that the aforementioned clergy would not arrive within that time. From this, I inferred that you parked when and where you did as a matter of convenience (which is about you)—for you could have parked elsewhere and walked the extra distance, or even returned to this fundraising sale to try again later, alternatives that wouldn't be as convenient as this. I also inferred from your opening post (OP) that you saw the sign and knew what it meant. Inferences are not assumptions, Josh.
Inferences can be wrong, of course, and if they are it's on you to indicate where and how—I can't know an inference is mistaken unless and until you tell me—but it would be less than rational to say something like, "Don't infer that my motives were selfish." You can't reasonably tell someone to not draw an inference.
And the reason why I inferred a self-centered motive is because nothing in your OP indicated that your choice was for anyone else's benefit. You wanted to stop then and there, check out some books and probably buy some, and this parking spot was the only one available at the time. (Please observe that I didn't say it was "entirely" self-centered. You may have had secondary or tertiary motives but they were not included in your OP—the only thing to which I had access.)
Don't assume, for example, ... that there was total disregard for ... the purported "property rules."
I neither assumed nor inferred a "total" disregard for the property rules on your part. Your OP indicated a disregard for an express property rule and I accepted that at face value. My response neither universalized the disregard nor needed to.
(I have noticed a tendency in your objections to deflect criticism by misrepresenting it in order to say it doesn't apply. For example, I criticize something as selfish and you respond by saying, "You shouldn't assume it was entirely selfish." But I never said it was "entirely" selfish. And here we have another example: I never said it was a "total" disregard for the rules. Please represent my criticisms accurately when addressing them.)
Consider the possibility any property rule itself had intended but unstated application that don't exist when either the pastor is not present or ... [snip rest]
What justification exists in your scenario to second-guess the meaning or intent of the no-parking sign? There was none. If the elderly woman had acquiesced to your logic, or if the aforementioned clergy had been called and agreed with you, that would serve as such a justification. But as the facts on the ground stand (given in your OP), the sign and its intent was unambiguous.
Any congregation led by a pastor demanding his or her space at the expense of an addict or couple in need ... [snip rest]
See? Here, again, we have this exaggerated language ("demanding"). If there is a pastor demanding his space at the expense of an addict or a couple in need, we have no evidence that it's at this church. (For example, do you know for a fact that they don't cover the sign during AA meetings?)
All we have here is a church that reserves a parking spot for its clergy, a priority which everyone appeared to be honoring—until you.
Lastly, what if the pastor was the one who told me of the book/bake sale and told me to park in her spot?
Was she? Did she? If so, you should have included that information because it would completely change my answer.
If she wasn't and she didn't, then it's irrelevant.
[If the pastor is the one who invited me to the fundraiser and told me to park in her spot,] were I to disclose that information to [the elderly woman] it might cause distress for her and disrupt her relationship with the pastor.
I am noticing your interesting word choice there ("might"). Which scenario do you think is more likely? Give your reasoning.
Woman: "You can't park there. It's reserved for the pastor."
You: "I know. She invited me to come and told me I can park in her spot."
- Woman: [Becomes distressed, anticipates tension with the pastor.]
- Woman: "She did? Sorry, carry on." [Thinks nothing more of it.]
- Woman: "She did? Sorry, carry on." [Goes to call pastor, who confirms.]
- Woman: "She did? Wait here while I confirm." [Goes to call pastor, who confirms.]
I will readily acknowledge my deliberate decision to disregard the sign's statement. The question is, "Is parking in the pastor's designated space sin?" Since scripture, not any one individual, is the definer of sin, I think the closest anyone will come to an affirmative answer to that question will be Romans 13.
Two things.
First, Romans 13 cannot be pressed into service here. Paul is speaking of civil magistrates, a context evidenced by his reference to penal power, public justice, and taxation (vv. 4-7). Scripture addresses other spheres of authority elsewhere (church, family, employers, etc.), and how we are to treat our neighbors versus the family of God. To flatten all authority under Romans 13 is exegetically careless.
Second, given the relevance of scripture for defining sin, what does it say about putting oneself ahead of others? As I said earlier, "Christian ethics is not utilitarian (outcome-based)"—so the likelihood of the pastor showing up is irrelevant—"but aretaic and teleological; the standard is not outcomes but Christ-centered obedience and the formation of Christ-like character." Do your words and conduct in that scenario lean toward or away from Christ's pattern of self-sacrifice, emptying himself for others (Rom. 15:1-3; Phil. 2:5–8)? The sign is incidental; what matters is how you responded to the church's rule and that elderly woman—who you call an accuser (so the self-justifying tone is also notable.) By your own admission, you deliberately disregarded the sign. It was a willful choice to elevate personal convenience above the stated wishes and priorities of the stewards entrusted with the property.
A general caution: To reduce sin to an explicit statutory violation would be inadequate and inconsistent with scripture. Think of the Sermon on the Mount, where Christ exposed such reductive thinking by equating lustful thoughts with committing adultery. Yes,
hamartia is "lawlessness" (1 John 3:4), but the law is more than the Ten Commandments—it is the whole covenantal will of God (Rom. 13:8–10). Consider the contrast between the letter versus the spirit of obedience.
Had my accuser practiced the Philippians 2 text ... Imagine how the conversation would have gone had I accused the accuser of idolatry

instead of legalism.
The question concerned what you did and whether it was sinful, not what she did.
... she'd have considered me and my purpose more significant than her need to defend the pastor [and her parking] space ... and allowed, if not applauded, my use of the space ...
I think that speaks for itself. Nothing more needs to be said.
It was fear, not respect, that kept others from using the space.
This is SUCH a telling statement. Overlooking the fact that you can't possibly know what motivated their compliance—which could be respect, or deference, or common courtesy—your framing betrays your own attitude toward authority.
I also cannot help noticing that you explained your disobedience situationally (convenience, utilitarian) but the obedience of others dispositionally (cowardice). You trivialized the rule ("absurdity") while impugning the compliance of others ("fear"). Why didn't you explain their obedience likewise situationally (respect, deference, courtesy)? This asymmetry is telling.
I guess if obedience is cowardice, then disobedience is a cut above.
Would [my supposed sin] be minor enough to assume the position of God and judge another?
I beg your pardon? Dude, YOU ASKED: "Is it sin ...?"
If people answer no, it's fine, but if they answer yes, it's judgmental?
The fundamental purpose of the OP is not to acquire a simple answer to the specific question asked based on personal opinion, but to provide for us all (including myself) an opportunity to reason with scripture through what does and does not qualify as sin—to better establish or hamartiology through what is otherwise a fairly innocuous example.
—which is precisely what we are doing here.