Well they sugest 50Kb or less for fast load times.
http://pages.ebay.co.uk/help/sell/photo_tutorial2.html
You could scan at high res and then resize the image to shrink the file size but I would think that people would prefer to be able to see detail.
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How about using a low res image but have a line in your sale description offering people the chance to email you for more details and a higher res image of the item. You could then upload images to a host and link to them from there or attach an image to the reply email.