I think that unless the person marks the cel 
NFS (and perhaps clearly says "don't email 
me about NFS sells...)

I think unless they do that, you might as well 
email them and ask.  People should be 
flattered when they get offers. I mean, what the 
heck, even if its a NFS cel, they may be in 
need of money one day, and remember that 
you offered them an obscene amount of 
money for that cel, and then email YOU!
who knows!

I just think there is NOTHING wrong with 
emailing someone an offer, unless they 
explicitly say "don't".

I'm flattered when I get offers (as long as 
person doesn't ask me more than once...) and 
sometimes I will sell a cel I wouldnt have 
normally becuase I got a really good offer.
real life and house payments sometimes can 
make that difference in whether I cel is NFS or 
not. ;)