I noticed in that group there's 34 sellers. So perhaps that's a rough estimate of how many sellers there are on ebid total from the USA. As for Buyers I'm not certain I just hope there's a larger pool of potential buyers. Ebid has 8 Million unique Visitors compared to eBays 62 Million. That's closer than I would have thought.
http://www.statista.com/statistics/2...rs-to-ebaycom/
http://en.wikipedia.org/wiki/EBid