Stolen art and watermarks

I guess it's my own fault really. I was naive enough to think that the only thing people would do with my art is maybe print them out and hang them on their walls or such. I was stunned today while walking thru the isles of the Star Wars Celebration 5 convention in Orlando to find someone selling prints of my work for $20.00 each. Due to this I have placed and will continue to place those ugly watermarks on all my work posted here. I know that will not stop those who have already downloaded and profit from my work but I found it to be necessary just the same. I am sorry.