You should have detailed close-up photographs of the drone before you sold it. There is no history of cracks just happening.
You'll probable just have to work out a deal with the buyer and follow eBay policy.
Actually, flying fixed wing single engine planes is not that simple as you have control of multiple parameters. If you pull back the stick/yoke you change the angle of attack and slow the airplane and descent, as in the flare when you land or...