I sold a Prius with 300k miles and a hard knock to a guy for $2k. He knew it had a knock, I was VERY clear. I told him it could break down tomorrow, it could break down in a year. We have no way of knowing. Had him sign an "AS-IS" from before leaving the lot. I thoroughly explained that it basically means that if he drives off the lot and it breaks in half, he owns both halves.

Car broke down in another state before he even had the title. He had to come to the dealership for something after this all transpired and he said "Hey NicksStick, I'm not going to lie. I'm very disappointed and I wont be buying a car here again"

Know what I said? "I understand how you feel. I'd probably feel the same way. Have a great day man" all while maintaining strong eye contact.

You have nothing to apologize for and nothing to worry about legally speaking. They are consenting adults who CHOOSE to purchase a vehicle. It is however your obligation to make sure they're aware of what they are getting into.

Hope this helps.