You have hit a seller's block and this could be for a number of reasons. Sellers can block people who have unpaid strikes, don't have a credit card on file, don't have a linked paypal account, aren't registered in countries the seller posts to. You could also be on the seller's blocked bidder list.
The seller is not obliged to remove the blocks or provide you with any information, it is even possible to block messages from people on the blocked bidder list.
