Animanga is one of the most respected dealers 
around.  I'd also recommend AnimeChaos, Animegame, 
 Japan Animetime, Anime Asylum, and Nichibei.  I 
have yet to deal with a bad anime store listed on 
the "Keep Track" portion of this site.

As for having cels appraised or verified, I don't 
think such a service even exists.  If it does, I'm 
sure the cost is only justified for very expensive 
American cels ($1,000+), not for $50-500 anime 
cels.  The simple way to verify a cel's 
authenticity is to find the exact scene in the 
anime and compare (but even that isn't foolproof). 
 You just have to buy from respectable dealers and 
have faith.

Here is my favorite Animanga purchase.  Aisha 
pumping up, about to kick Gene's butt (after he 
bought her dinner no less!).  ^_^

JP

PS---Welcome to the club!