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