Many people make the mistake of scanning or photographing at high resolutions and then using those images for the web.
Truth be told, you really do not need to exceed 72 dpi for the web. Anything of a greater resolution just takes an externity to appear... especially if you are on dial up.
Don't confuse resolution with image size. If you are going to scan the stamps your best solution is to scan at 300 dpi. You can then use a program like PhotoShop or PhotoShop Elements (which ships with a lot of scanners) to "Save for Web" which will chnage the resolution from 300 dpi to 72 dpi without changing the image size. Trust me - you will not see a visible difference on your monitor. 300 dpi should only be used for actual printing and never for the web.
As an example:


The top image is 300 x 200 pixels at 300 dpi (file size 95.2 kb) while the bottom image is 300 x 200 pixels and 72 dpi (19.7 kb). Now if your images are substantially larger in size at 300 dpi you may find that some potential bidders get fed up waiting for your images to upload.
Hope that helps a bit.