you should be able to buy whatever you want for 
any reason.  even if the reason for buying those 
cels is because you have money burning a hole in 
your wallet.  perhaps you might like the chacter 
design without seeing the show or movie.  maybe 
you heard of how good the anime was and wanted to 
collect in hopes you see it in the future and 
actually do like it.  its a free country, and if 
your not in a free country, then you have free 
will.  do what you like.