The best thing you can do is use sites like alexa to see traffic volume of competitors.
I did this and narrowed my choices to Bonanza, Ebid, and Ecrater.
So far Ecrater seems to turn out the most sales; I am averaging about two a weeks. However feedback is rarely given back on that site, and I find that it is rarely known. Ecrater ranks highest out of the three for total traffic. And they have recently reached over 2 million orders. Something pale in comparison to ebay, but the fact it is in the millions shows you it does have some selling potential.
Ebid is another one I used, sales seem to come and go, with months going between them. I keep items I don't care about, and know won't sell well on other platforms on that site. I also don't like that they charge a membership fee. This fee seems like the company itself doesn't make money off selling items, but rather selling memberships, and a fake promise of sales. They even offer a 1 week trial for 1.99 which is the biggest waste ever, you will not make a single sale in that week, and they are just trying to get as many memberships as possible.
Bonanza is one that seems the most promising, it has the cleanest and most original interface, thought their traffic is garbage.
Basically there almost is no alternative unless you go into niches.
If you sell stamps bidstart is a good platform to sell on. Otherwise there aren't a whole lot of other places.
It is unfortunate that ebay gets to monopolize an entire market and no one has bothered to compete.
I have even gone as desperate as trying to join foreign sites like ebay, but they don't allow North American sellers; sites like Raukuten or whatever it's called.
Alibaba is launching a site called 11main; rumoured, for the Western world, or North America I guess.
Don't know why Yahoo Auctions ever failed; not enough advertising.
BUt basically the biggest problem is people already have Ebay drilled into their brains as the only place, so it's hard for anyone to step in.
And if someone is going to step in they will have to be a corporate giant, that has the means and advertising abilities to do so.
I think if someone bought Ebid, or Bonanza, and revamped their platform and promoted it heavily it could be done.