Sounds like it spanked him so he stuffed it away wet then sold it. You might have a shot at him for not as described? But to be honest a few dirt stains won't hurt, unless it's oily or poo-ey.
Hopefully the scrubbing with soapy water hasn't hurt it any. Might affect its water-resistance a bit though.
Anyway, you're not alone in disappointing kite purchases. I once bought a pair of kites from a guy off another forum. One of the kites had a bunch of minor repair patches that he did tell me about (landed in a bush, he said), but the other had two completely blown cell walls. Pretty serious internal damage that I spotted as soon as I launched the kite but he claimed he didn't know about it. I was so not amused.
Happy ending is he paid the repair costs and the kite still flies

in fact I was flying it just a couple of weekends ago in Northern Ireland.