I have trouble finding anything *cheap* in my size (xs) on ebay. By the time the bidding closes, the prices are the same as I would pay for something off the sale rack at the LBS. Add to that shipping and the lack of immediate gratification (Iwant it NOW!,lol), and I've become disillusioned with trying to get apparel that way.
Anyway, to answer your question, you'd probably wear a men's small. I personally don't care for "unisex" jerseys. Women's jerseys have narrower shoulders and the princess seaming enhances the performance of the fancy performance fabrics by keeping them close to your skin.
As I've looked for jerseys in my size, I've found that I pay the same buying off the local sale rack or from the sales from online retailers as I would for something off ebay.
HTH