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.