As Don mentions, you never want to resize an image upwards, only downwards.
For
ebay, I scan so that the widest dimension is 1000 pixels to 1600 pixels.
Since I always put images in the body of the listing in addition to using the uploadable image feature (the ones that show up at the top of the page in
ebay's rollover image viewer), in order to make sure the body images don't expand past a device's width forcing people to scroll everywhere, in the HTML code I add a WIDTH="XX%" parameter. This automatically resizes the image upwards or downwards to fit XX% of the width of the containing object, thus making the image responsive (or at least as responsive as it can be within
ebay's page code).
By starting with a high-resolution image, if the image ends up being resized in any direction, it is more likely to be reduced in scale from its original size (good) rather than enlarged (bad).
Code example:
<p><img src='www.placeyouthosttheimages.com/imagename.jpg' width='100%' alt='Description'/></p>
(note: I normally would put https colon slash slash in front of the www above, but it gets auto-mangled by the forum software even inside the
code tag.)