the most trustworthy place would be the BMW used car outlet (Niederlassung) at Fröttmaning by the football stadium. It is a BMW owned sales outlet that only deals in used cars, most of them ex-BMW employee cars but also many trade-ins. Don't expect to necessarily get the best price there, though.
You can use this locator to find a used car from the "Niederlassung".
Alternatively you can try and purchase a year old car directly from an employee:
there are two links on that page on the bottom right: one where you can try and locate cars currently available and one to where you can configure a car that you'd like and make a contract with an employee who will then buy the car, run it for the agreed time/km (minimum 6 months, i believe) and then sell on to you.
I'm not sure where you'd go if you wanted older BMWs but there are plenty of used car sales lots on the road out of town on Landsbergerstrasse/Bodenseestrasse