As well as the items suggested by jensgram, it is also important to consider the type of milage it is likely to have, and how well maintained it will be.
A large saloon with a high milage is likely to have been a rep-mobile - it will have done most of its miles on the highway, at high speed. On the other hand, a smaller, older hatchback with very low miles will probably have been owned by an older person, and only used for popping down the shops. IMHO the latter is worse - without being used, seals dry up, parts perish more quickly etc, and maintenance is more likely to have been neglected. On the other hand, a sports car with a low milage is likely to have been a cherished second car, and will have been looked after more carefully.
I normally work on an avarage, for a UK car, of around 10-12k per year, so if a car has done a lot more or less than that I'd be more cautious about it. 50k miles in less than a year seems like an awful lot, and I'd suggest that was probably a commercial/fleet vehicle, wheras 30k in three years sounds quite normal to me. Obviously this does depend on the average milages in your area though.