why dont you just talk to person A about it?

say something like "I really need the moneY" 
(which is the only reason I can think of that you 
would sell such a gift cel...)
and ask them if its ok if you sell it. reaffirm that 
you really appreciated the gift.

I think it depends on how much effort and time 
the person put into aquiring this for you.

I mean if someone bought me a $100 cel I 
probably wouldnt sell it. ;)

though... I dont usually sell gift cels anyway.

I wouldnt be offended if someone sold a cel 
that I gave them. its a GIFT and its theirs to do 
with as they please. if they sell it, it makes me 
happy becuase they obviously wanted the 
money, so I have done something nice for 
them, whether it is the cel, or the money the 
recieve by selling the cel.