The first thing to understand is when, as a matter of fact, you need to sell. There are, of course, objective and subjective factors, the priority of which should preferably be given to the former.
The main signal for sale - the car starts to "fall apart" or is restored after a serious accident. In the vast majority of cases, the car is sold exactly due to these reasons. If your runs for service became more frequent up to unacceptable values, it means it is time to part.