I've found that much like everyone else, I've 
learned by experience, watching auctions and 
dealer prices, and through the advice of friends.

Ultimately, it comes down to how much you want 
the cel and how rare it is.  If its something 
that you've seen pop up fairly frequently, then 
you can feel more assured about waiting for the 
right one and right price.  Right meaning the 
price/terms you're comfortable with.

OTOH, if its something really rare, I usually 
snap up the chance as soon as I can.  I make what 
may seem like ludicrous offers to some, but I 
know its the right price for ME because I'm happy 
for the opportunity I might not have had 
otherwise.  Those super rare cels may only come 
once in a blue moon- so I've found that if I 
don't act quickly, I may have a very long wait 
ahead of me or more expensive.  I've lucked out 
in finding some cels privately when sellers had 
to sell for whatever reasons that otherwise would 
not have, even for a lucrative offer.  I 
generally avoid auctions which can get pricey and 
make me feel anxious about the sniping, dealing 
with deputy services etc..  unless theres 
something I really have to have on there.

But ultimately, it all comes down to what you 
feel comfortable with.

~ miwa