Ebay user screws me!

So I sold a lens on Ebay the guy got it about a week ago. Today I get this e-mail.

Hi, I purchased your 50mm 1.4d lens. I just came back from overseas and opened your package and checked out this "mint" and i was a little disappointed, the front glass has a 3/8"-1/2" some flow in the glass and I was looking for a MINT lens. So let me know who you wanna handle this. Thank you. Tim

The lens was "Mint"

Does anyone have a clue what "the front glass has a 3/8"-1/2" some flow in the glass" means? What should I do?