I've never tried the bulbs from that ebay seller, but most of the ones that I've purchased on ebay I haven't been too impressed with. I've tried 14K and 20K bulbs from 2 different sellers and both bulbs were closer to 10K than what they were advertised as.
A few years back I found one decent ebay seller that carried Reef Optics bulbs. I was impressed with these bulbs, but they were not as cheap as most of the other ebay bulbs. Pretty much identical to XM bulbs so you could always try them for yourself. He no longer sells on ebay but here is his online store:
http://www.reefopticslighting.com/bulbs.html