I'm no halachic expert, but I don't understand how a kinyan can be made on something that does not exist. By not exist I mean something you did not buy yet and something you may be unable to buy.
Furthermore, if the dealer never specified WHICH coin he's paying for, i.e. from which account the order has to be made, the buyer of the coin can always say that the coin he purchased was not part of the agreement and that he intended to purchase a different one to fulfill the agreement.
You're focusing on one of the (at least) three problems with PFS's business model that prevent him from having a halachik claim. Retail seller didn't have the object yet, no fair market price existed, and no payment was made. Depending on how many of those problems you resolve, there might be a mechusar emunah issue or even mi shepara, but in the current business model none of those apply.
The bottom line here is that PFS had a business model that never worked al pi halacha, but up until now it didn't matter. But with the frenzied competition doubling and tripling his offers this time (that we've never encountered before), and with his lagging response choosing to match (never beat) upon request (which is questionable ethics in its own right), most of his potential buyers saw a tempting reason to abandon the deal. After all, why bother reaching out to ask for a match - at best he'll match it, and if you happened to not see the best deal out there he's not going to tell you about it. I'm sure based on this experience he'll have to consider changes to his business model for future.