I searched this and i didnt find anything so i'll post it:
On my server darkmoon faire cards go for around 50-200g each but a whole deck sells for 1500g so basically you can buy all the cards in a set, make a deck and sell it for a lot more then what you bought it for. You can even farm the aces of the deck to make more of a profit if you want. I've tested it before and its made me quite a lot of money. Also I suggest selling them on fridays or weekends or whenever majority of people are on to almost garantee they'll sell