eBay auctions showing a single chip or token work fine at 400 dpi. That resolution is only used by their software when the bidder uses the "magnify" option to look at a piece of your picture magnified. I save mine from Photoshop at quality level 10 which applies very little compression when saving your raw scanned image. That results in a saved file that's from 150-200 KBy in size and it uploads in about 1 second with my internet service.

Remember, 100 dpi on a chip that's about 1 1/2 inch in diameter gives you a 1 1/2 inch display picture of your chip on most monitors. 400 dpi will give you a display 4 times that (about 6 inches) on most monitors. eBay's software magnifier only lets you look at a fraction of the image as you move the magnifier around the chip so you never see the complete 6 inch chip at one time on your display. You do see the piece you're looking at magnified 4 times, though.