I think you must be 18 years or older to be a 
legal eBay user because binding contractual 
agreements cannot be enforced on minors, 
certainly not those done without the consent of a 
parent or legal guardian. (I'm not sure about the 
legal ramifications if the adult gave consent.) 
When an auction ends with a winning bid, both 
parties consent to complete the transaction in an 
honest manner. My guess is that eBay would rather 
not deal with irresponsible minors being involved 
in their auctions by stating in the registration 
process that you must be at least 18 years old. 
eBay can't force the guilty party to do anything; 
the most it can do is to permanently suspend 
his/her account.

---Trebuchet