I think that the car is still under warranty from BMW.  All new BMW come with 3 years or 60,000 kms BSI (BMW Service Inclusive) which mean that 99% of the repair will be cover by the dealer including oil change.  But you have to do everything on schedule (Can't go and change the engine oil before it's due).  Ex. First oil change on E60 can take up to 18,000-20,000 kms depend on driving condition, weather and etc.  These criteria will be monitor by the car's onboard computer.  All of these details that I have given you are from my experience of using BMW E60 (Been using it for 2 years now and before that I was driving E39 for 5 years) Yes I'm always a BMW fan. 

 

I don't know why the owner of this BMW want to sell his/her car, maybe you can call and ask him or her directly.  I think that there is also another listing on the classifies for the similar model in red color. 

 

If you decided to buy it, you can always take it to the dealer for full check up. (Little peace of mind that you or the seller can pay for)