Try www.amazon.co.uk they have something called amazon marketplace .
If you have a item to sell they put your item next to there listing, you get paid direct form amazon. To the age thing i am not sure, but i have used amazon market place a few times and as a buyer it is alot easier. Amazon is a very good site you should see a padlock at the bottom of your screen locked and that means that no one can get your credit card number, anyway most credit card companys have a anti fraud guarantee for online buying etc mine does.