I would guess it's to weed out scammers. I also can see where a lot of adults would have an issue entering a sales contract with a minor, as if the deal goes south the minor legally can't be held to it.

Most buyers also like to use PayPal to pay since they get some protection that way, but the account has to be linked to a bank account - again, not something a minor can have in his or her own name. So it may be best if you handled the financial part of the transaction and your daughter did the rest of the work, such as taking pictures, writing the ads, etc.