Mileage is an important factor when considering a used car, but it is not the only element to consider. As you say, a car of around five years old with 75,000 miles will have covered around 15,000 miles a year, which is slightly higher than average but not excessive, so it is likely to have covered a fair proportion of motorway miles which are generally less stressful on vehicles than urban driving. A car with above average mileage but that is in excellent condition and priced appropriately is arguably a safer bet than one of a similar age, lower mileage but that is showing signs of neglect and a patchy service history.