I can tell you what distinguishes the people selling a lot on FAA, or any similar site, from those who don't sell. Mostly you won't see the evidence of the difference by looking at their galleries. True, in part it's the quality and nature of the work, and how well it suits the customers who frequent the site in question and what they're looking for. Perhaps it's a little to do with price point though not in the sense that "cheaper is better". (Advice -- don't necessarily lower your price if you're not selling. Instead, find out where the customers are who will buy the work you want to make at your asking price. If the answer is nobody, nowhere, then maybe you have a problem. Conversely if you find the right customers for your work, you can probably raise your prices, depending on where you've set them now.)
More than those things, the ones who sell a lot are the ones who don't confuse uploading an image to a print-on-demand service with marketing & selling work. Instead, they're the ones out there promoting, networking, marketing and selling in pretty much every way they can. And who are committed to investing the time needed to build the audience for their work, because for most of us it's not a 90-days-to-fame proposition. If you upload to FAA and sit back, you'll almost certainly sell little or nothing.
Your best bet is to treat FAA as a selling infrastructure service provider -- they host the gallery, provide the shopping tools, print, frame, package, ship, handle the financial transactions, deal with customer returns, connect through other online avenues like Google search & Amazon, etc. The free account is actually useful, and even the paid account is so cheap as to be next to free. But don't think it won't cost you something. It will cost you the effort to actively find & drive customers to your work. Just having a gallery on FAA won't promote, market or sell your work for you.
I'm on FAA myself, and like the service for several reasons. I sell enough to pay for the annual fees probably for the rest of my life if they stay at a reasonable level. But in all honesty it's not a scrap of the level of business I could be doing through the site if I was a lot more actively promoting my work showcased there and directly sending customers there to buy.
