I would use the scan's that other people had 
created.  Overall must previous owners would 
either not mind the recycling or be in favor for 
it.  In numerous sites you see people who put up 
graveyard links, where they show the cel's they 
used to own.  you keeping the original scan of 
the cel, and the watermark would be a further 
example for the people who have graveyard cel 
links, kind of saying i had that before but i had 
to part with it even though it is beatiful, kind 
of thing.

In the end, you may have several people who are 
disgruntled by you placing the cel up on your 
site, but then again you can always take those 
particular cel's off temporarily while you make 
your own scan, so this way you will only have 
maybe one or two cel's to rescan.  I would put at 
the top of each page that you where using other 
peoples scan's but that you owned the cel's know, 
so if they wanted a trade or to purchase the cel 
they could ask you and not bother the person who 
you bought the cel from.  Or you could just put 
this information, in BIG BOLD PRINT, on the front 
portion of your site, so that everyone would 
know.  I don't even have a large collection and 
it took me several day's, (altogether) to get all 
my cel's up.  I can't imagine having to do it all 
over again, blah blah blah blah blah blah blah 
blah blah blah blah blah blah blah blah blah 
blaaahhhhhh . . . Blah . . . etc . . .