Bikes are an easy (and cheap) way of getting around in Saarbrücken. Since the environment is quite hilly, you should buy one with gear-shift.
Every second Saturday of the summer months there is a flea market (Flohmarkt) near the Alte Brücke in Franz-Joseph-Röder-Straße. With a bit of luck you can find a second-hand bicycle there, as well as all the other second-hand things you might need, like clothes, chairs, books, etc . The market starts early in the morning (good time for bargains) and ends in the early afternoon. On the same day, a bit later in the morning, there is a second-hand bicycle market on Tbilisser Platz in front of the Staatstheater. The bicycle market is organized by the ADFC. There you can find a cheap bike from 50-100 Euro.
The usual price for a new bike starts at about 400 Euro and goes up to whatever you are willing to pay (the higher the price, the better the bike…). There are several bike shops in town, like Der Fahrradladen, Nauwieser Straße 17, where English is spoken. They sell second-hand bikes, too. You can also try a bike shop near Westspange (Tel. 0681-49318).
When you leave Saarbrücken and the MPI-INF and you don’t need the bike anymore, you can sell it to another student or offer it for sale at a reduced rate at one of the above-mentioned markets.