There are several reasons you may be prohibited from bidding or buying an item due to ebay's seller settings which allow for placing restrictions on buyers and/or their accounts.
The first is Unpaid Item strikes.
Another is your location, it may be the buyer doesn't ship to the country you reside or to Post Office boxes or something like that.
Also, sellers can block buyers without Paypal Accounts associated to their ebay ID.
Or if you have bid on a certain number of their items already.
These are most of the restrictions that come to mind. Only you can know which of these might apply to you.
