Well, there are actually a few ways to get golds ingame... You can spamm REP quests or Mission Orders (Dungeon Missions that gave Experience and Golds), farm materials with your sprite and re-sell them, and such things like this.
If you buy APs... Well, my advice is to always give a look at the current Tier Spender
(Note: There's usually a bonus one on the weekends, so it's a Double Tier Spender on Sat/Sun) The Tier Spender will give you bonus items when you spend APs.
Regarding selling AP cards, well, that's at your own risk
, so always be carefull if you chose to sell AP cards. Here
you can find a list of the currend Trusted AP buyer/sellers.
Hope it helped a bit /o