There are ways around making a direct monetary
offer that can help if you are interested in
someone's cel. 

First, you could just email them saying "Hi, could
you keep me in mind if you ever sell ____________?
Thanks".  I keep a mental note on most of those
emails. I'm sure other collectors keep track of
these too.

And there is always the "Is _______ up for offers
right now?" To just get the idea if it is or isn't
- if you're not sure. 

~Tara