I just sold a car in France and the buyer was running around in it for a fortnight and returned to the garage because the heater wasn’t working on the left. The garagist said that in France a buyer has three weeks to return the car in default so I fixed the heater for €285 and all continued. In this case, if the garagist is correct, and he is a friend, then the vendor must fix the problem or pay for it to be fixed.
How did this end? I just looked up this thread because I have a similar problem, but with a mountain bike. It is still a vehicle, right?
I bought the bike from a rental shop on a ski-field. We agreed that I could pick it up in 2 weeks and that they would do a full service and replace the tires. When I got there they had covered the bike in a bleeding hard to remove black protective tape. Taping up a bike is common practice to protect the frame. I did my best to inspect the frame then and couldn’t find a fault. So, I bought it. This was the end of July
In this winter’s break from cycling, I have been removing the tape and lo and behold, there is crack in the frame on the rear triangle. Yes, I know , I should have walked away or demanded they remove the tape, but unfortunately, emotions took over from cold rationality 
Can anyone give any advice?
It’s going to be tricky to prove the crack was there in July.
I know. C’est Moche!
It’s still under warranty, I just need to convince them to do it,