I have been selling on eBay since 1998 under various "niche" accounts and stores.
I sold anything from:
Vintage clothing: This was a very hot niche for me in the late 90's to the mid 2000. I did especially well in the "EMO" style niche. But when I moved to France I pretty much dropped that niche because at the time the French postal service were not offering any cheap small packet rates. Now that they have been fined a couple of millions euros by the European Union for this aberration, and are now offering small airmail packet rates I guess could go back selling vintage clothes. But for what I see the market is not as strong as it use to be. So I still offer the occasional vintage t-shirt or French designer piece, but that's pretty much it.
Collectibles: all sorts of collectibles and small antiques. Stamps, coins and paper money as well.
Magazines and books: especially French magazines and rare French books
Now back to the "never accept a buy now offer on an Ebay auction" AKA "you know you got a winner item when....":
If you list an item up on auction and within a few hours you got messages asking if you would sell for a fix price, it pretty much mean you hit the jackpot.
Never accept any offer for a buy now. Even if it sounds attractive. It just mean the buyer KNOWS it will go high and he tries to get a deal. Even if the offer sounds attractive (like 10 times or 20 times your starting price).
I had this happen often:
One time it was a vintage Iron Maiden t-shirt. Within a few hours I got 3 offers ranging from 50$ to 125$. I refused them all and the t-shirt sold for over 300$ to a Japanese buyer.
Another time it was a very "ugly" advertising display for some sort of drinks. Some plastic sculpture of a guy holding a tray of said drink and the display would be illuminated by a light bulb. Ugly. I paid 2$ at a garage sale (I mean for 2 bucks, the risk of loosing money is almost nil). Same, got a bunch of offers, turned them all down and end up selling the thing for 500$ to someone who's dad had a bar in the 50's with the same ad. He wanted it as a tribute/souvenir of his father.
So if you don't know what it's worth, let the auction run its course. On eBay or elsewhere!