I'm guessing you'd lose a lot of the resolution though, wouldn't you?
Yes, but it's better than nothing.
Really? That's strange. On Macs you can capture an on-screen image and you lose none of the resolution. (None of the DISPLAYED resolution that is - if the displayed image is 300ppi then yes, you'd lose that, but since the images on eBay are for records only, you wouldn't them any higher than your monitor can display anyway. In other words, 72 or 90 ppi will remain 72 or 90 ppi forever, however many times you capture it with a screenprint.)
It is input resolution that is potentially lost, not output resolution (which is by definition fixed by the display). As an example, if you have an image that is four times the width and height of your screen (in pixels) then to display it full screen, only 1/16 of the total pixels are used to generate that image. A screenshot will therefore lose 15/16ths of the available data - therefore resolution has been lost.