I've sold a lot of things on Amazon. I like it better than ebay because it's easier to list an item, and I like it better than Craigslist because I don't have to interact with people. I usually ship stuff myself. Last time I looked at FBA it didn't look cost effective for what I was selling. Sometimes I will take their trade in offer if it's a good deal or if I'm lazy. Usually I'll take the trade in offer for books if it's reasonable (not pennies) since I can ship them a batch of stuff.
A lot of sellers, including Amazon themselves, will adjust their prices to try to undercut everyone else. If you want your stuff to sell you often have to go in once a day or more and set your price back down to be the lowest. For most things I'm selling I don't really care how much I get for it, I just want it to sell. It's kind of fun to undercut the market and completely devalue what you are selling. I listed one book where the price started out at $75. I got into a low price war with another seller, and they would always lower their price to a few pennies below mine. So I would go in and lower my price by a few dollars! Eventually the price of the book dropped from to $25 and it finally sold.