I partially aggree with Jinxy; deplomacy is the 
answer,
though making an outright offer on the person's 
private collection,
in my honest opinion is a no-no.  The way I do it 
is I tell 
the person how much I love his gallery (gotta use 
the butter-up method).
Then I ask about the one cel and mention that if he 
would ever 
consider to sell it, that I would love to buy it.
Maybe he will ask me what I want to pay, maybe he 
names his own price, 
maybe he just says OK and maybe he ignores me 
completly.
Either way, you won't put the gallery owner on the 
spot and
you won't embarress yourself.

The Evil Jew