The simplest answer is to place your name or auction ID or website address on the photo so that it covers part of the image without obscuring vital details. The software you use to view/edit your photos almost certainly has a text feature (contrasting text color on a transparant background works well..)

A. if someone uses material you do have a copyright on you have proof it was used when it shows up.
B. anyone who uses your photo is driving lookers your way, especially if you use your website address.
C. cropping your name out of the photo will destroy its usefulness as a display of the item.