The NPC who Looks Like He Knows Something provides the following helpful speech if you speak with him.
Hey, Character. You found the Black Market. Goody for you. I hope you didn't spread it around too much, or we gotta do another midnight move.
You look pretty green to this buyin' and sellin' stuff so lemme explain how it goes down. We use a 'secret bid' auction. You set the price for what you wanna sell, but the Buyer can't see it. He goes and bids the most he's willing to cough up and if he wants to pay as much (or more) than you asked for, you got a sweet deal with a profit and he gets the item. Now just so's we don't totally overcharge here, we got this system that shows a 'history' of how much that particular thing's sold for in the past. He can decide how much he wants to spend.
Now listen here, you only got a limited number of Transaction Slots so whether yer sellin' or buyin', you can only do a few transactions at a time. Got that? It's important. In the beginning, you only got a few Transaction Slots. Sucks to be you, right? Don't worry, it gets better. You get more through experience. The amount of slots you have is based on your Threat Level.
You can sell (or buy) Salvage, all Enhancements, Recipes, and Inspirations.
Now this service ain't free, pal. You gotta pay to play so listen up. You got your Listing Fee, which is 5% of the price you sell your item for. Don't worry, we'll give it back to ya if your item sells. You also got your Transaction Fee which is 10% of the final sale price. The 5% you paid when you listed the item is subtracted from the Transaction Fee. Now ain't that nice? You're gonna use your Infamy to pay for all these wonderful goods.
Now get this straight. The minimum listing fee is 5 Infamy. We gotta make a living, see, so if you can't move your goods, you lose your listing fee. It's just business. Anyway, you can always put up a 'looking to buy' order. The stuff you're selling will appear indefinitely, as long as you show up once in a while. If you got AWOL for 60 days or more, they're gone, pal.
As our way of sayin' thanks, we've got special badges for being a power seller and some of 'em even increase your Transaction Inventory size. Sweet deal, yo.