Generally speaking (there are some exceptions), flatbed scanners that do negs/transparencies do so by shining light thru the neg from behind, rather than lighting a document/print from the front (normal flatbed scanning). To do this, they usually turn off the main scanning light, and activate a light in the transparancy adaptor lid. In my Epsons case, the normal scanner lid gets replaced with another lid with what looks like a 4x5 lightbox in the middle of it. When it scans in neg/transparancy mode, it turns on this light but not the main light. I put the negs in little holders (came with the scanner) that keep them off the glass at the optimun height (1-1.5mm) but it also works just laying them on the glass. Mounted slides just get laid on the glass. The color MF pic above (last one) was from a unmounted transparency so I put that in the holder (the film strip just slides thru)

So, if you searched ebay for a Epson 1640, you want to make sure it had the TPU (Transparancy Unit) and preferrably the neg holders. You could buy the same scanner without the TPU. Not sure if the same applies for the 2450

Hope this helps, but you'll be too busy playing with the Bronnie to do too much eBuying