I am glad that you got an apology.

I don't see how she could have thought you were a
non-paying bidder since it only took her 5 days to
sell it to someone else.  Last I looked, ebay
states that you have to wait 7 days.  Also, I know
for a fact that it can take up to 5 days or longer
for mail to get from one place to another..

I can understand being paranoid about NPB but you
have to at least give your buyer a chance to
complete the transaction.  She didn't even offer
any alternative for you.  Like Spinizuey said,
when you buy or sell you enter into a lawful
contract.  You can't just let your auction end
with an acceptable bidder and then turn around and
decide to sell to someone else. 

Either she got a higher offer or she needed the
money faster.