well legally you have to be 18, but thiers loads of people who just use it because its a quick way to earn money and recently they added a thing where if you list an item thats 99p or less as a start bid, its free
To get money from people you set up a 'paypal' account ..
www.paypal.com ...then have to go through the process of linking the ebay and paypal account together. Then you link the paypal account to your bank account to be able to transfer funds
It comes as Cash in your account, depending on whether the person pays by paypal or not ... but Paypal is the easiest and safest.
Thiers also the whole thing about ebay taking 'fees' on a final sale item aswell ..and paypal doing the same but thats probably alot of info for now lol