The selling page is supposed to refuse your data if it doesn't meet the new resolution requirements. I've found that 400 bpi works fine although you can use higher if you want more detail to show. When someone browses your auction, hovering the cursor over one of the pictures will automatically enlarge it; you don't have to enable anything else.

A trick I found is if you've already listed a similar item, you can use the "Sell similar" option on the old auction to open a new auction with all the options and text that you had in the previous listing. You can then just type over what is different and when you're happy with the new one, just submit it. This gets all the same categories for your new auction as well the the same formatting you set up for the previous auction. Just remember to delete the images from your old auction and submit the new ones in place. This has nothing to do with getting a hi-res scan into an auction. It's just a short cut that simplifies the creation of a number of new auctions.