This does happen often, and not just with cels.  Not every
dealer is familiar with lots of anime series, so if it looks
like something, they figure that's what it must be.  It's
especially common on eBay or other auction site.  Very often
you can snag a bargain on something that's mislabled, though
lots of times the misidentification means the seller may 
think it's worth a larger base price than it really is.  
The only thing you can do is politely contact the seller and
say "I believe you're mistaken, cel X is actually from show
Y, not show Z" or such.