Kat, m'luv, I subscribe to several used book news lists, and since bibliofind was folded by Amazon and many of them are about to go with zshops, etc. on Amazon, almost all of them are complaining bitterly about the process of uploading their inventories and the difficulty of getting any help or advice from Amazonian staffers. Some say they will go to any other listing service (ABE, BookAvenue, etc.) before going to bolster Bezos' balderdash.
I look at eBay books almost every night, just in case one or another of my current wants might appear, but I always check the prices being asked by professional booksellers before I bid. I pop over to Bookfinder.com, check the prevailing price range, and then go back to eBay and bid only if it appears to be a truly competitive price. One must always be careful not to bid on remainders that some sellers pick up for a song and then turn around and fly on eBay with a slightly higher starting price than the remainder cost them. I look through the Hamilton Books listings in my areas of interest, and try to keep those listings in mind while I'm scanning eBay.
I've recently seen a web site where books for sale can be listed free. Can't find my bookmark for that one right now, but I'll hunt it up and let you know.