Is it a good idea to buy a BMW?
In respect to this, is it worth buying used BMW?
When purchasing a used BMW, it's important to be aware of the depreciation. Yes, BMWs are awesome cars, however like most vehicles, they lose their value quickly. BMWs can be more expensive to maintain than non-luxury cars and older model parts can be hard to come by.
Also asked, why Used BMW are so cheap?
They lower the price. Now if one person lowers the price of their car, everyone has to lower their price because no one will buy their car if there is a cheaper one out there. That makes the price go down since people are afraid to spend on a car that they think might cost them a lot of money down the road.
BMWs get a bad reputation for reliability and quite unfairly. There is good reason for many peoples fear of German cars in general, but generally BMW and Mercedes have been pretty solid cars and, at least used to, have a good reputation. If you maintain your BMW it will last forever. At least the engine will.