It appears that in Dominion speak, "a copy of card X" is basically shorthand for "a card (any card) with the same name as card X." It does not mean "a clone of card X," and it is not the opposite of "card X itself." In fact, any time you see a named reference to, e.g., Curse, you could always replace that with "a copy of Curse" and not change the meaning. "Copy" is only used when name of the card being referenced is contingent on gameplay, rather than a set part of the card's effect, but should otherwise operate the same as calling a card by name.
I can't think of anything wrong with saying that Buy targets a specific card in the supply, and then gains you a copy of that card from the supply. The Gaining part of buying is only retrieving information from the targeted specific card, not moving it, so it's allowed to track accurately even if the original target has changed location. That makes Talisman work correctly, even despite strange timings.