eBay holds their trademark very closely - a search for eBay in the WIPO or ICANN judgements show that they have no problem going to arbitration to get a hold of a domain name that infringes upon their trademark - and that wouldn't include all the cases where a webmaster handed over an eBay-related domain name after receiving a cease & desist letter (before it would go to WIPO or ICANN).
Some companies don't hold their trademarks closely when it comes to others using it in a domain name. But eBay is one of the ones that keeps a very close watch on this, so it wouldn't be too long before you'd hear from them, if you did decide to go ahead and register a name with eBay in it.
