I think a lot of the trouble is that many expect far too much from these new operations, they're not ebay and they have absolutely no chance of generating anywhere near the volume of traffic that ebay does.
Having said that, I completely agree with you Don, far too much promotion of the site, and many like it is left up to the actually sellers themselves. Sure, as a seller you should be taking every opportunity to market every selling platform your are on, by way of any means possible.
Not to do that is not covering your market, and leaving gaps is when you start having sales slip through them. However, while these sites rely so heavily on their members doing their promotion for them they have a limited life span.
Sooner or later members get fed up and pull the pin, start losing your hardcore support base, and the whole platform will then struggle to survive.
I always look at a site like Bonanzle as a pretty good alternative for someone just starting out with the strategy of developing their own website. OF course it doesn't provide you with the flexibility that your own website does, but it essentially the same style of platform, and it's FREE to list!
I think it's a great platform for those to cut their teeth on, little bit of work involved, but zero risk!
Take advantage of their platform to gain some experience before you run with your own site. Test out your online marketing skills, learn how to drive traffic to your booth and all the other aspects related to online sales.
Good little learning ground, once you have a plan in place, you know what you're doing, and you're ready to leave home. Then you can go out and get yourself a place of your own, and you're not starting from scratch.
If you want a solid platform to make constant sales from, then these types of platforms will struggle to provide you that at this point of their development.