Did they say what was not described about it? They could just be after a freebie. Maybe if they have low feedback, there main account has been busted for shoplifting so they've started a new account. If it wasn't expensive, they may think that it won't be worth your while to have it returned so will just refund them.

I would be asking for photos of whatever it is that's not as described. I would also be wary about asking eBay to step in as we all know that 99.9% of the time, regardless of who asks them to step in, they immediately close in favour of the buyer.

I would be asking for a photo, then ringing eBay and telling them that the item is as described and you feel the buyer has buyer remorse.