Just upload your pictures once. If you've tagged them well, they'll show up whenever potential buyers search for what you have to offer. For maximum exposure, you'll need to take advantage of FAA's marketing and promotion tools (Facebook postings/shares, Facebook shopping cart, image links, etc.) YOU are the one most responsible for promoting your work. FAA provides a number of excellent tools to help you, more than any other service I know of, so take advantage of as many as you can.
The markup is what you'll receive if you sell a print. It can be any amount you want. FAA's printing costs are added to your markup to make the price the buyer sees.
If you want to order your own prints, sign into your account then purchase them normally. Your markup will be removed (just for you) and you can buy your own prints at FAA's cost.