Everybody would understand either one as meaning that there were very few sold. If I had to peg one as being completely grammatically correct, I would choose "sold hardly any copies", because 'hardly' really modifies 'any'. While everybody would understand "hardly sold any copies", it could be strictly interpreted to mean "just after they sold any copies." Again, almost nobody would notice the difference.

Both of your sentences are grammatically correct, however, it is more natural when speaking English to put an adverb like hardly in front of a verb it is modifying. For example, I would find it much more natural to say, "I quickly ran to the store" than to say "I ran quickly to the store". Really, it is just personal preference and it doesn't matter.