if they are being sold as production cels.  AIC 
made a lot of them and distributed them to the 
public.  As Chi said, multiple xerographic copies 
were made of some of the nicer sketches and 
painted, typically with slight differences from 
the cel used for the video that are fairly easy 
to spot once you find the frame on the original 
source material.  

Of interest, almost all of the repros on ebay 
over the last couple of years have been from OVAs 
4 and 5.  I have occasionally seen repros from 
the other OVAs on Yahoo Japan.