As stated multiple times already, it is entirely up to the seller to decide:
Who he whats to sell to,
How much he'll sell it for,
Where the transaction is made.

There are No entitlements. A Posting or PM doesn't give you the 1st right to buy something (unless that is what the seller said in his ad). But with Pm's there is no way to know if you were truly 1st anyway. It stinks when you think you were 1st and they sell it to someone else (happened to me several times), but that's just the way it goes.

As far as the ethical/integrity side of it, the only thing that is a big deal to me is if you back out of a deal that you already made. But if one or the other party wants to change anything about the agreement (price, time to meet, place, etc.) then that's up to them. You are only (ethically) required to keep the terms of the original agreement.

Example: Person A is going to sell an item to Person B on Monday. Person B says they now can't make it, so how about Wednesday. Normally, Person A should/would agree to that if it worked for them. But I don't think they're tied to that any longer. Person A may need that sale done so they can buy something else they have set up on Monday night (etc.).

Overall - this site is pretty good. I've been pleased with basically everyone that I've done a deal with (partial exception of 1).