Perhaps the cards should be tradeable via their own trading interface. Make it to where cards can only be traded for other cards. This could be extended, as well, to additionally help combat RMT: Make it impossible to use the TT trading interface outside of Gold Saucer, and similarly impossible to trade regular items or gil while inside Gold Saucer. Place restrictions on trading so that people can't just trade a * card for a ***** card by immediately registering cards given to them and disallowing the trade of cards already owned by the recipient (ie, you can't give someone a Moogle for a Bahamut if they already own the Moogle). Additional restrictions could be in place to limit what ranks/rarities can be traded (ie, you can't trade a Moogle for a Bahamut, even if your trading partner didn't have the Moogle already, as the former is common and the latter is rare).

This would give a market for duplicate cards but help mitigate RMT abuse.