Regarding going after buyers, if potential buyers know that buying a counterfeit item puts them at legal risk, I think the market for said counterfeit items will shrink substantially. I think that could be a potentially convincing argument in the case of items where the buyer clearly knows (or should know) that the item they are buying is counterfeit. Is it really all that different from receiving stolen property, which IS illegal?
I think in the FTDI case, though, it would be really hard to argue that the end users should (or even could) know that the chip they have is counterfeit.
What would FTDI be able to go after them for? The end-user isn't copying anything so they can't run afoul of much with regards to copyright law, they know they're getting a counterfeit so they're not being fooled by trademark infringement and probably only care about the functionality of their ICs anyways, and the internals are completely different so the patent risk is minimal.
I think in the FTDI case, though, it would be really hard to argue that the end users should (or even could) know that the chip they have is counterfeit.