I think that what you've done-- putting together 
a page with illustrative screencaps-- is an 
excellent start. It works very well for 
characters-in-general; if you're on good enough 
terms with a seller to refer them to your 
wishlist, they may not be familiar with the show, 
but they may recognize the similarity of a 
screencap to a cel they have, and direct you to 
it. That's what happened to me recently. ^_^

But it helps to be on very good terms with the 
seller, and have a history of being a strong 
customer. And even if I might think I'm a strong 
customer, they might not think so, and not bestow 
the 'perks'. ;o) So for the most part, the burden 
of filling your wishlist lies primarily on 
yourself, making the rounds and keeping an eye 
out--- especially when you start narrowing down 
from particular characters (or particular 
characters in certain forms) to specific scenes 
or even a specific frame within a scene. 

Thoughtful friends are also an immense help. :o)