The existing marketplace solution for that is prepaid credit cards, essentially gift certs that can be redeemed by being run as a credit card. Teens use them to shop online, or at least my kids do.
There is also limited liability with prepaid CCs, no matter what happens you can't be out more than the value of the CC, but you can get into considerable financial trouble with paypal WRT overdrafts and overdraft fees and bounced check fees ... its just not worth it unless you're well disciplined.
