Ehh, it's a toughie, because both sides can be 
argued.

If it was a thanks-for-your-business omake cel, 
I'd probably have very little hesitations in 
selling it off to make room for something I 
really wanted. But with that extra personal level 
of I'm-your-friend-so-here's-a-cel-that-I-liked-
but-I-got-it-for-you... that makes it a bit more 
difficult because not only did someone buy a cel 
that they liked... but they passed it along as a 
present (and ergo, a loss) to someone they 
*liked* enough to give up something special to. 
Consequently, the cel suddenly aquires a personal 
level of meaning.

It gets difficult, because it would be incredibly 
cheap to say, "Here, you can buy this back from 
me--- it's the gift you gave me! Remember it?" 
But by the same token, if you *are* getting rid 
of it, it's probably because you need the money 
the cel is worth, more than you need the cel 
itself. 

If you do decide to sell, and if you have it on 
display on your website, you might quietly take 
it down and not mention it to your friend and 
hope that it never comes up in conversation. 
Because, unless you tell them that you sold it, 
they wouldn't know... and knowing will probably 
hurt the relationship. Then again, so would lying 
to protect their feelings, if they tripped you 
up. ;o)

If it was me, I'd probably hang onto the cel for 
an indefinite amount of time, and get rid of 
something with comparatively little personal 
attachment. I could probably make up the fundage 
elsewhere, unless someone was making a private 
offer of hundreds of dollars for that particular 
cel only. And if someone *was* doing such a 
thing... you should consider yourself very lucky 
to have such a generous friend, and think about 
the consequences/implications of your actions. ^_^

No easy answer! Sorry!
-Cres