You do not need to upgrade your account to a seller account if you want to sell. But if you wanted to upgrade your account, then you will need to pay (I think a basic shop is now £14.99 a month).
When you look in your "eBay account", eBay usually takes off the money someone has paid after an item has been sold. When they do this, you need to press a button on the page which says "make a one-time payment".
See above point on eBay, Paypal also take a share of the money too. This can be seen in your paypal account.
You report the problem to eBay. In addition, if it was a 360 game, I'd post it recorded and get proof of posting. You could state in your description that you would not be held responsible for the game being broken in the post. Mind you, if you did post it, you would obviously pack it very well.
One thing though, I've been having problems with eBay. As a seller, I'm not able to leave neutral or negative feedback, as eBay thinks it helps buyers be honest etc. So be careful, people do take advantage of small sellers. I hope the above helps you.