The repairs were done at a service, don't know exactly where/what only the total amount. What I know is that the car didn't have any obvious issues when I've sold it since I drove it a many times recently and it drove really well. I don't think the repairs were mandatory but more like improvements.
First thing you should do is ask him for a specified bill which clearly states what all has done/repaired/replaced on the car and what all of those things bare for individual costs. Than you have to decide which costs are legally yours and pay those. If he does not come with this info, you do not have to pay him.
I would think that service is a consequence of ordinary wear and tear, including most replacement parts used.
It's highly unusual to sell a relatively inexpensive used hand between private people without exclusion of warranty. "Ab Platz" or "wie gesehen" is almost exclusively used, which excludes all warranty. Since it's unclear if you owe any part of the bill at all (I think you don't, your landlord would think likewise if you ordered work in a rented apartment without prior Ok by the landlord) I'd try to use that to posthand eliminate warranty.
But for that you need to play tough at start, block everything. You need to prove you'll be a pain to deal with, thereby making it attractive for the buyer to agree, not so much from a monetary standpoint but because you are a huge pain to deal with.
Indeed, if he took it in for a fluid change/checkup and the mech said "The pads are nearly gone and the discs are worn enough that we might as well do them when the car is on the lift (and since we're at it, let's do the wheel bearings)" then that shouldn't be your liability.
If the car overheats after a 30m run in stop and go traffic, then fair enough, that's a genuine fault.
Bring me pretty much any 10 year old car, even fresh after the MFK, and I can easily find 1000CHF worth of stuff that it would be nice to have renewed, without actually being a "fault".
If sold privately without a garantee- not your problem. Anyone who buys second hand without a garantee, and without getting the car checked by a good mechanic- has only got their own eyes to cry (as we say in French).
Here is a contract, in French but available in all national langs- to protect you when you sell a car second-hand privately- that states clearly it is sold as seen and no responsibility from seller in case of future problems.
If I remember correctly either autoscout or TCS have the wording of a sample contract that should be used when selling a car, so you don't get these potential issues. eg. to exclude the warranty thing