I've had a similar problem with some rims.
Was told there was only a mark on the front, turned out to be a couple of dents and one in the rear.
Told the seller I was disappointed with the condition but happy to keep them (without sounding aggro).
He offered a full refund but that's a waste of time because I'd have been out of pocket on the post.
I suggested a partial refund. We agreed on 50%.
Be upfront with the guy, he might be cool about it. Especially if you have to leave feedback