Further to what Padi said, the actual buyer requirement states "Block buyers who are currently winning or have bought X of my items in the last 10 days". X being anything from 1 to 100. Note the word "winning". There is also the option to only block people with 5 or less feedback, but that obviously doesn't apply to you.
Does the seller have auctions and you have bid, regardless of whether you have won? That's all I can think of.
