Here's how a buyer can be blocked:
1) Are registered in countries to which you don't ship.
2) Have a negative feedback score.
3) Have received Unpaid Item strikes.
4) Are currently winning or have bought '1-100' of your items in the last 10 days.
5) Do not have a PayPal account.
http://pages.ebay.com/help/sell/buyer-requirements.html
If none of these apply, and you feel the seller is reliable, then contact support using this link:
http://support.auctionsniper.com/