Legally, you could also claim you never saw the p.m., even if they could prove that you got it, you could claim you didn't read it.
Failure to act cannot create a binding contract, btw. Also it's not true that if you offer something for sale, and somebody offers to buy it, that creates a contract. It does no such thing. Sellers reserve the right, legally, to sell to anybody they want. Also an individual can even go so far as to discriminate against someone for any reason! You can say you don't want to sell it to somebody because they're hispanic, or something equally as screwed up, and get away with it.
There's certain laws that limit that, like in real estate transactions, but most laws about things like that only apply to business with several employees. Someone selling something privately can sell it to whomever they want for whatever messed up reason they want.
Well, you seem to know a lot about the law. Show me a caselaw study where a guy sold an arcade game pcb to somebody, discriminated against another buyer, and was convicted in court over it.
Some shit is just common sense interpretation.
If that was solid logic then I could say, "show me a case where a guy sold an arcade game pcb to somebody, discriminated against another buyer, and wasn't convicted." The law doesn't work like that. Facts fit into certain legal frameworks and then those frameworks are applied to the facts to give a (hopefully) consistent result.
Common sense can be a good guide, but in this case I think your common sense is fundamentally at odds with contract law.
Acceptance (via mail or e-mail) typically occurs upon delivery, not when you read it. With the mail, there's even a "posting rule" which says acceptance happens when you mail it. I won't get into all the logic behind this but it has to do with being able to resolve "he said she said scenarios". Invariably someone lies about when they got it, or whether they read it or whathaveyou and the law has wisened up to shit like that. With PMs it's not hard to discern delivery because it's seconds after the PM is sent. Judges are smart too. If you said some shit like "well I got it, but I didn't read it" or "I got it, but there's no way to prove I read it" then you'd be toast. You'd also be exposing yourself to felony perjury charges, so you've gone from civil liability for breach of contract to potentially losing voting rights. So always tell the truth. Also, you should never use the phrase "get away with it" when dealing with the Court system.
Okay, also, failure to act can create a binding contract. But instead of diving down that hypothetical rabbit hole let's deal with the facts. By offering something for sale, you HAVE acted. You've made an offer. The question then arises: what sort of offer have you made?
You CAN reserve rights in an offer. BUT, you can only reserve rights by actually reserving them. Like, if you say "I'll sell this PCB for $50.00, but I reserve the right to sell it to the purchaser of my choosing, or not to sell it all". That sets up a limited offer, and it's really more of a request for offers by purchasers than an offer on your behalf. If you just say "I'll sell this PCB for $50.00" that reserves nothing. That's an unqualified offer. If a buyer says "I accept" you have a contract. If he's willing and able to pay and you fail to sell, you've breached. You can't get cute and say "I'm not selling to you" and then when he doesn't pay say "well, we've both breached". The law will look at the first breach only, (for obvious reasons.) Now, if there was some well accepted custom in the arcade parts realm that there is always a reservation of rights in all sales then that custom might work to create a rule, but I doubt you'd get enough consensus to create that sort of customary rule here. So, in sum, you have to actually communicate to people what your intents are to fully protect yourself. And that's a good thing anyway because it prevents customers from being pissed about something that could be prevented.
Race discrimination opens up a whole new can of worms. The only advice I can give you (disclaimer, no I'm not giving you legal advice, just common sense advice) is (1) remember that you're dealing in interstate commerce and (2) racism is just plain wrong.
Again, all of this is completely and totally overblown since these rules apply to multi-million dollar transactions the same way that they do to $50 transactions. They only usually matter in large transactions because hey, it's just not worth it for $50