Without commenting on the specific facts of this transaction, I think people have a mistaken understanding of the law of contract. Assuming US law on this is the same as the Anglo-Canadian law, what you need is offer, acceptance and consideration. In the context of a cue sale, the consideration is the price that is agreed to be paid in exchange for the cue (it does not have to be paid yet for the contract to be binding). You generally do not need an invoice, a signed contract, other any other formality. The following link suggests the law is the same in at least one US State.
http://jec.unm.edu/education/online-training/contract-law-tutorial/contract-fundamentals-part-2
Unless there is something material to be agreed on, or terms imposed via custom of the industry or something like terms of use of a website, or there is consumer protection legislation in place (for example allowing people a cooling off period from door to door sales etc., or requiring certain types of contract to be in writing and/or signed), buyers and sellers both can be bound to a deal though a verbal agreement or an email exchange.
I'm not suggesting that people should be suing each other over cue deals, but I do think people should understand that there can be legal consequences for "changing their mind" in these circumstances.
Gideon<---Not an American lawyer. Anyone considering their legal position should get actual legal advice