Lots of things, really. Here are the main ones:
Make and model - some cars are just more desirable than others
Age - newer cars are generally worth more
Mileage - lower mileage usually means better value
Condition - bodywork, interior, and mechanical condition all matter
Service history - full service history adds value
MOT length - more months left on the MOT helps
Number of previous owners - fewer is generally better
Colour - yes, really! Silver, black, white, and grey tend to be mor...