Oh Dear.... I do a lot of art work for my groups/clubs and although I sign, I generally also inbed copyright dates and backup on zip/cd. Copy right infringment can and is seriouse buisness! :(
You should be covered under international Copy right laws Goldy and I would keep track of all those sellers useing your pic, get one of those cd's and see if any more of your work shows up on it, plus it would be absolute proof one way or the other if he has or hasn't used any others. Then contact ebay for removal. If they don't, seek legal help.