Most auction sites recognise this as an offence, Image and Description Theft Policy

QXL Terms & Conditions

6.5 Prohibitions

As the Seller, you are responsible for ensuring that the sale of any Item, the Listing itself, and all activity or communication (including any posting of a Member Rating) you make in connection with any Transaction will not (a) be capable of infringing the intellectual property or other rights of any person or entity, (b) breach any applicable law, whether criminal, tortious or otherwise, or (c) be offensive, obscene, pornographic, insulting, false, unreliable or misleading.

Much of the early growth and development of the internet was by achedemics who were looking for ways to share information and copy protection was not something which was seen as a necessity in the programs they developed. The web browser and HTML were developed by these same people and it is virtually impossible to stop people from copying your item descriptions and images, but it is possible to make it more difficult.

A simple thing which you can do to your images is to add some text to them, i.e. your user id, you need to place this prominantly on the image so that the text cannot just be cropped from the image.

An alternative is to disable right clicks with Java Script (right click on this page to see this and compare it with other pages which are not protected).
Click here for the code