Two other things to note:

Don't start the offer request with "I'll give you 
foo dollars for that."  "Would you accept foo 
dollars for this cel?" at the end of a message 
explaining that that was your dream cel sounds 
much less confrontational.

Also, don't be afraid to ask if there's anything 
the person would be willing to accept in trade.  
If you've got anything good (even not necessarily 
cels--ask if the person collects other related 
goods, like toys or doujinshi or books) that you 
think they might be interested in owning, say 
"Would you be interested in trading?  I've got..."  
A lot of private collectors are much more 
receptive to trading cels than selling them 
outright, especially for higher end/more desirable 
material.