As far as buying a MB on ebay I would avoid 1) New sellers/members, 2) Those that don't have a high detailed seller feedback score and 3) those that have few transactions. Long time members know the value of doing right by their buyers since one bad rating can destroy their credibility. I have personally sold a 2009 MB, 2009 iMac, and a 2010 13" MBP and I think part of the reason is that I have been a member since 2005 with a perfect rating. The type of person I would buy a high end item from needs to meet the above criteria else I would just wait for the right item with the right seller profile.
Also, make sure you pay with paypal since that is the only way to resolve a problem if there is one. I have had 2 or 3 issues with sellers and I was able to get those problems addressed to my satisfaction through ebay.