There's lots of good advice here Pale so I wish 
you good luck. But let me add a couple things:

Even if this person turns you down, make sure you 
express the comment that "if you ever decide to 
sell or if you need cash fast, you've got a buyer 
here!" I think most people who have been 
listening to an honest offer in the first place, 
would be willing to consider you in the future. I 
think some people have received cels they never 
expected to through this route.

Also I agree that you've got to offer a price to 
what the cel means to =you= PLUS the going market 
rate (that means, know your prices, kiddo!).

Of course the downside of telling somebody how 
much you like their cel is that you might 
reconvince them that it's a great cel. In some 
negotiations, I've actually refused the offer 
because the person had resparked my love for that 
particular cel (terrible isn't it?).

And as for trades: look closely at their 
wishlist. 

At the very least, this person might be able to 
help you find a similar cel in the future since 
he or she is obviously interested in that 
cel/series. And you'll mostly likely have an 
interesting correspondence with someone of like 
taste so enjoy your discussion.

Good luck! And if you get it, be sure to show it 
to everybody ^_^ (unless you need to respect the 
collector's wishes of anonymity).

Wendy