On the surface, I totally agree with kenshinlvr 
that for someone to offer a higher price for an 
item that is already "sold" in the hopes of 
getting the seller to renege on the initial deal 
and sell it to him/her instead is totally wrong.

But let me add this wrinkle.  Although I'm new to 
this cel collecting, one theme that I keep hearing 
OVER and OVER from dealers is "most cel collectors 
back out of deals".  Maybe it's the mad rush to 
"hold" cels as soon as they get updated, so you 
don't miss out.  Or maybe there are many young 
collectors who simply can't afford all the stuff 
they order.  I don't know.  What I DO know is that 
many sellers don't consider a piece truly SOLD 
until payment is received.  It's the nature of 
this particular business.

Knowing this, I can see where someone might 
step up and say "IF the original buyer flakes out 
on you, I'm very reliable....." (I did this myself 
just last night).  But it is still borderline to 
offer MORE in the hopes of disqualifying the 
original buyer.  (Case to ponder: How much time do 
you give the original buyer to remit payment, and 
how wrong is it to say, "I really LOVE this cel, 
it is worth X dollars to me, and I would LOVE to 
buy it if the other buyer is jerking you around?")

Until all the buyers act as professionals, I can 
certainly see where the sellers have some leeway 
to protect their interests.

All in all, a sticky situation.

JP