Wow, that sucks! Had a look, the website took a long time to load.. I would not know that that person had taken away the watermark, so I guess people would easily just fall for it as it's a great piece. Makes me wonder if everything is stolen on that site.. Did you contact the site? Hopefully they will remove it asap, and didn't make any money of your art..
Yes was my second guess that someone else stole it and send it to them for sale. Already send an email with screenshot and wonder if they react...
Guess you made it when your art gets stolen...I hope they didn't make money :(
Always makes me angry when I see someone else art get stolen and I always contact the artist.