Yes, it happens. A LOT!!!

Just check out celsforsale.com  Everyone of those 
Tenchi cels were for sale on other sites. The 
pretty sammy, for example, came from Nichibei and 
I've seen all the others elsewhere.

Mangaya does this too I believe. I know that 
Black Lady with the tennis racket was in Anime 
Taro. 

Maybe the dealers are losing their sources? Or 
maybe they're just getting a lot greedier. 
Anyway, I didn't buy that Black Lady at $110 and 
I sure as hell aren't going to pay $200 for her 
now. 

p.s.- Black Lady's on page 24 of their site.