The problem with what Drac offerred you to do by
keeping the payment or keeping ANY payment is that
the seller "can" be liable for a law suit if the
buyer decides some time in the distant future that
they want their money back.  

I know that the buyer was very irresponsible at
not sending the sufficient funds, but according to
the law (they tend to protect the buyer A LOT more
than the seller), the buyer is entitled to a full
refund UNLESS they signed a contract which stated
that "If you can't comply with so and so terms,
then you will lose the money".

Example - I purchased a $2000 ticket for a trip to
Korea 2 years ago.  After all the news with the
conflict between N and S Korea, I decided that it
would no longer be safe going there.  I signed a
contract 2 years ago saying that if I back out,
then I lose the deposit of $500.  Well, when I
told them I wanted to back out, they refused to
give me ANY of the money.  Then, they wanted to
give me only $400 back because they say they have
already purchased the tickets, etc, etc (this was
from a Ticket Agent place which menas that the
ticket WAS indeed refundable and they lied about
it).  Well, after I was about take them to court
over that, the lady suddenly denies that she ever
"refused" to give me my $1500 and i ended up
getting it back.

Now, THIS example is very similar to your
situation in that the reason why I lost the
deposit was because I agreed to the contract.  The
seller was in the wrong because she didn't want to
give me ANY money back which went against the
CONTRACT.

So, to sum up everything, IF YOU DIDN'T AGREE BY
CONTRACT ABOUT LOSING THE PAYMENT, then the buyer
will be able to sue you for their money back if
they wanted to.

Please take this into consideration when making
your decision.