Good point. If you suspect this is even a remote possibility, it won't hurt to edit your QRZ bio to say something like, "I am NOT selling anything at present. If you see someone using my call to sell something, do NOT buy. I'll delete this note if I ever offer something for sale". On second thought, regardless of whether you suspect a scammer has hijacked your call sign, you could add a note to your QRZ bio saying, "Please drop me a quick e-mail telling me what prompted you to look me up on QRZ." That might give you an answer to the question.