The best way to use your AP, would be to make sure your character gets well-established first. Make sure your enchants are ready to go, sockets, etc. Plan out what you want your character to do for every 10 levels that you reach (after 30,) and compare your set goals to the ones you already accomplished. If you failed to meet your goal, you can use your AP to compensate through the next level bracket.
Selling your AP in-game for gold is a relatively tricky business. You need to keep an eye out on trends, and sell your items at the right time for the best profit. Selling AP card pins is even more tricky, unless you find a well-known and trusted buyer that's willing to walk you through it the first couple of times. At least, until you get used to it.
As for selling items that you get with the AP: Magic Alchemy Clays normally tend to go like hotcakes. Especially when the clay table is good. Other items that normally bring in a decent gold profit: Fortified Sage's Stones, Master's Fortified Inscriptions, prefix scrolls, and other direct-gear-enhancing items. But, remember to watch the trends to ensure that you pull in the largest profit from them. At the same time, you want to keep an eye on tiered events to make sure that you can get even more value per point, and sell the items for an even greater profit margin.
As for other ways to make money, if you can't get AP, or have a slow day selling your AP items:
- Run the 5 MO money-making limit on every dungeon that you can. It's relatively balanced in the payout, per level bracket. It may seem slow and small, at first, but if you save your money, it can build up quick.
- Level your Guardian Sprite's skills. Normally those items sell for a decent price, especially if they have a direct connection to making valuable yellow-grade items. Salvage works on green-grade and higher gear, and those items can fetch a relatively decent profit from time-to-time.
- Make sure to do as many quests as you can! Not only do quests give a relatively decent payout (with many of them being repeatable,) you can also gain city/faction fame (which give you access to certain items and discounts,) but titles, and they generally give a fair chunk of XP on turn-in.