Unfortunately, other destinations (e.g., the supermarket) may be unreachable or inconvenient, entailing a transfer somewhere. You may still be able to do without a car if you make arrangements for those routine trips, for example, with a housemate. Still, you should probably consider buying a car; see below.
A good part of the year, bicycling is a good alternative; you may even combine bus and bicycle, as many buses are equipped with bicycle racks in front (check with PVTA for routes). When biking, use common sense, as most of the time you will share the road with cars, and many drivers are not prepared or plainly do not see bicycles. So wear bright clothes, use lights when appropriate, use a helmet. Outside the downtown of Amherst you can ride on sidewalks, off the street; there is also a bike trail joining Amherst and Northampton and extensions planned.
When parking the bike (there are racks outside our building, but do not use the handrails), also use common sense and good locks: theft of bikes is very high in the area, especially on campus. Should you want to bring your precious two-wheel into the building, be considerate of others, and do not drip water and mud, do not leave bikes on exits or other fire sensitive areas, etc.
Buying a car is tricky, so be patient and ask for help. Private car sales are advertised in the Hampshire Gazette (the Amherst Bulletin is free and carries the same ads, but appears only weekly), the Auto Shopper magazine (free and available in campus center), the local for-sale newsgroups, and various other places. Pleasant Journey in Northampton seems to have a good reputation selling used cars, mostly Toyotas, but ask around for recommendations, and read the Consumer Reports annual Auto issue (available at the Jones Library; ask at the reference desk).
The approximate price of any used car appears in the Blue Book . Another useful source is Edmund's . You can find out the reliability of a car model in Consumer Reports (mentioned above). Make sure to get a ``pre-purchase'' inspection done by a mechanic of your choice before buying a used car, and if buying from a dealer, get any warranties in writing. Consider also future maintenance and insurance costs; on used cars, they may quickly dwarf the purchase price. It might make sense to buy a new car under certain circumstances; but in either case, do not overextend yourself financially.