How should I estimate used car value ?

I have a 10 year old diesel with about 230k kms. Has been a trouble free car for 7 years but recently two valves (?) popped out while driving and I had to get it towed. Was told it would be 2-4 k to repair depending on damage. Looking online a running car line this is listed for 6/7.5k.

Garage has offered to buy it but what a fair value be ?

Kicker is that the car broke down and was towed to a garage in Kildare but I am in Cork. I think a guy at the garage will buy it privately and work on it in his spare time and then sell on.