Cels Forum

The best thing to do is to check out The Internet Guide to Anime Cels (http://www-scf.usc.edu/~ckawakam/cels/) - they 
have a ton of links there, and a rating system of sorts.  
There are some sites that are no longer active, even though the pages are still there (Aurora's and Anime Lane), and 
there are some dealers who are known to be very very slow 
(Mangaya). I have also had good dealings with Anime-cel.com.

My understanding is that the best thing to do is do a search
here for a dealer's name, to see if anything has been 
written about them, or to just ask in the forum.  Also, I've
found that the time it takes a dealer to respond to an email
is proportional to the time it takes them to update sites, 
send out cels, etc.

There are others here probably better able to answer your questions though, so keep on asking!

Az