Damage after private sale

Hello MT-Gemeinde, please don’t scold or point out the SuFu, my concern is something more special! To the facts: Last year 10/12 dripped out of the plastic tank of my Aprilia Pegaso BJ. 2004 some gasoline. Then I expanded the tank and brought it to a specialist workshop. There the tank was reglued, I had paid almost 100€. I then built the tank again and left the motorcycle in the garage. I can’t be proven to be back with the motorcycle was driven, the tank was dry! For reasons of time I then inserted the motorcycle at mobile. I pointed out in the offer text that the tank was re-glued by a workshop. Two days ago then the later buyer was with me and looked at the krad. He drove 5km sample and how we agreed about the sale. During the viewing I pointed out again on the glued tank – this was no problem for the buyer. I had a standard purchase contract from mobile. A We forgot any reasons in the purchase contract to note that the tank was glued ! On today he picked up the krad and after about 35km the buyer called me and told me that fuel would run out of the area of the tank … How do I have to behave? I am an honest person, therefore I had also noted the lack in the advertisement. I just feel so fucked up, the buyer will not feel better… Please help me!