The same thing happened to me with a Lands End diaper bag. I listed it as "used" with some wear. The lady who purchased it wanted her money back and claimed it was "unusable"...well, I don't agree with that, but...
I ended up refunding her money, plus all shipping costs. Ouch. I was a MESS through the whole process. I take things VERY personally (bad trait of mine

) and then I started second guessing all of my used items, whether or not they are good enough, etc.
I have 100% positive feedback and I am very honest in my descriptions. I am considering putting a "disclaimer" though, that condition of used items is subjective and if you have a problem buying used, then don't bid" Something to that effect.
Also, I have sold a few things for WAY more than I originally bought them for. One was my son's raincoat. I bought it for 9.99 at Marshall's and sold it for 20.99!! It was hardly used.
The other were some tub toys that my mother purchased for my kids. They were about 5 years old or so. Elmo toys. She probably paid $10 or $15 for them new. I sold them on eBay for $31!!! It never ceases to amaze me what people will buy and how much they will spend!!!
Anyway, with that all said, since January we have made nearly $900 just from outgrown toys, clothes, and items that have been stored in boxes and moved a few times! So, with all the running to the p.o, and all the listing, worrying, etc....it is WORTH it!!
Marcy