Mileage is a very important factor in the retained value of a car like this. The more kilometres it has covered, the less it will be worth. There’s also a sort of kilometre-cliff over which a car will plummet in residual-value terms once it breaches that distance-covered.

Even though you’ve cared for the car and serviced it properly, most buyers will be a bit leery of a car like a Pulsar with so many kilometres on board. Even in otherwise good condition, it would probably struggle to fetch more than about $3000 in the current market.

It could be the battery; it could be the fuel pump. I had a similar problem that turned out to be a blown head gasket, so it could be worth doing a leak test.

I would top it up and watch it for a week or two. If it drops substantially then take it back and have it checked, but if all was well before the service there’s no reason to think anything has happened during the service to change that. Trying to prove the mechanic didn’t top it up properly is too difficult to attempt. If you’re unhappy with him go somewhere else next time.