This is too broad of an issue so unfortunately there is no way to give you a clear answer to your specific questions.
However, here is what I would suggest. Before you buy the vehicle have it scanned at a local parts store for all the codes and then supply those to us and we can get a better idea as to what the issue is. This will at least give us an idea on the range of possible causes. Then you will have a better idea of what to pay.
For that matter, I would definitely ask that the price be reduced if he does not have this fixed prior to you purchasing it.
If it were me, I would want to have it fixed and pay less for the vehicle then trust that he is going to have it fixed because he is going to take the cheapest route possible and depending on the issue this is not always the best thing.
Wednesday, June 10th, 2020 AT 5:34 PM