The problem is, in the US anyway, contracts are pretty much null and void for minors. They can terminate them anytime they wish on their whim, regardless of what it says, without any penalties that may be listed in the contract.
The probably applies to the ebay agreement (a contract), meaning he doesn't have to necessarily sell it to you even if you win the auction. On the other hand, I doubt he can just take a payment and not send the agreed item; if you send it, he'll either need to send the item or refund your money.