That is a question that I have found myself googling quite a bit. I found some statistics after hours of google search and I thought it’s worthwhile to share them. (I also replied to a quora question about it)

According to this article, in 2006 (I know, that’s from 6 years ago), eBay had approximately 1.3 million sellers using that platform as the¬†primary or secondary source of income. Out of that 1.3 million, approximately 630,000 are in the United States. Those results were published by eBay themselves and were part of a study conducted by AC Nielsen on their behalf.

But as pointed out by this Daily Beast article, you’ve got to pay close attention to the term “primary or secondary source of income”. Not all 1.3M are serious sellers on eBay.

The numbers I have for Amazon are more recent (3 years ago is better than 6 years ago, right?). In 2009, Amazon said that they have 1.6 million active third party sellers, up 19% from 2008. I would use the same argument as what I used for the eBay article above, and say that not all 1.6M are serious sellers on the Amazon marketplace.

If you have better information to update this article, please let me know (just comment on this blogpost)