I can't say anything about Yahoo Japan. 

As for my collection -- half from dealers, half 
from ebay and other collectors and I don't have 
complaints about any of them. They've all been 
great. 

Ebay and Yahoo can be both heaven and hell for 
collectors. You might find a real deal or a wish 
list cel, but there's the possibility of paying 
through the roof for them. 

Most of the dealers on cel.org are reputable. 
It's best though to check all of the archives to 
see if there are legitimate complaints against 
any of them -- cels simply not being sent, being 
packed poorly, etc. As a last resort you could 
ask on the forum about them first, but most of 
the time you're not going to hear anything bad 
about them. 

Good luck with that future Trunks cel.