Depends on the 'dream cel'.  If it's just of a 
regular character from an anime series, sure I 
could sell it.  I've sold a few cels like that 
where I've loved the character but someone 
offered just the right price to persuade me.  I 
can probably find the character in future, so it 
doesn't bother me.

But, if it's of a specific scene or cel I've been 
looking years for, it'd take a very high price 
and me just happening to want another cel (or 
something else really expensive) at the same time 
before I'd consider selling.  Mainly because it's 
rather unlikely to find cels twice from an exact 
scene that many other people probably love as 
well.

And so I give you the most common answer in the 
world: "It depends." ^^