Is it just this buyer you are having problems with? Maybe you are coming across one of their blocks, for example, some sellers will block those who do not have Paypal, or a credit card on file.
If you are a new user, you may just have hit your limit for this month on bidding, and can only buy Buy it Nows with an immediate payment for now.
Sorry not to be too exact, but all I can suggest from the clue you gave in your post.
