This happens alot, I have seen cels one or 2 
sequence numbers off from mine sell for more and 
several for alot less, it just depends.

I usually buy both also, I think it is wrong to 
not buy the item but If you really want out of 
the deal, you should email the seller and inform 
them that you are no longer interested in 
purchasing the first cel, You would forfit the 
downpayment and might make an enemy but most 
sellers will understand if you explain why
it would be worse to disapear and not answer 
their emails.

 ...unless this was an auction, I don't think it 
is ever right to back out of an auction because 
it is what you bid and the seller would be losing 
money with you backing out. Make sure that you 
get the other cel before you let go of the first 
one, because someone might have bought it and the 
website may not even have it anymore then you 
would be out both cels

just my thoughts on it ..good luck wither way