I don't think there are any sites that do that 
though.  Occasionally you get a seller online, 
but you really just have to watch this forum for 
that, because someone here usually notices 
something suspicious and brings it up to be 
verified.  Its usually a one-time thing.  

There is one cel seller though, who does a lot of 
stuff... studio something or other... they say 
they 'don't know' if their cels are real or not, 
but they're a collective of artists and also 
sell 'cel reproduction painting kits' so *cof* 
its suspicious.  just look up cel kits on ebay.