I use Auctionsniper.com I've never had a bid blocked through that...
It actually logs in and bids as me so I don't even know how they'd block it.
Is it possible you were blocked due to your region, feedback, or lack of paypal account? those are all ways in which an ebay seller can block bidders.
http://community.auctionsniper.com/e...1/m/7731090862
to give you an idea of how often I use auctionsniper... unless it's a buy it now/best offer auction I will place a snipe through the site and I probably buy 5-10 items a month through ebay.