Well, my right brain says to think ill of you 
(hypothetically) since the other person gave it 
to you as a gift, even tho they liked the cel.  
My left brain though (dust goes flying around as 
I havent been in there for awhile) says that it's 
your cel, you can do with it what you please.  
It's one of those odd things..it's really no ones 
business but your own.  In the past, excluding 
one incident, I have usually checked with people 
who sold me things before I sold them elsewhere.  
This is only for those times when sellers have 
expressed regret to me after they've sold me 
something.  In all fairness tho, it's your cel, 
and unless you signed a contract with Person A, 
you can do what you want.