Don't give up an "EX" quality lens that was rated "BGN" by KEH only because it has a minor scuff blemish on the front element. The scuff will not cause flare (doesn't work that way), and most likely will only be detectable if you happen to be looking at front of the camera, and not at the film plane, or captured images.
Forget resale value, as a "BGN" lens from KEH is a steal anyway. All that matters is that it works without issue. If you trade it in for a "BGN" lens that has no glass blemishes at all, you are likely getting a well used lens that had a tough life, countless shutter trips (for shutter equipped lenses), and shows it in terms of look and feel.
Solid mechanical (and electrical if it's the case) condition, along with high optical performance is of primary importance in reality. The scuffed lens that you described here sounds like a winner on all counts to me. Prove it's performance by testing it next to a known premium condition example before you return it if the scuff bothers you that much. Don't just give up a great lens deal out of hand.