Second-hand car markets in NRW

Hello,

I am interested to buy a second hand car. I have been looking through autoscout24 and mobile website. I am interested to know if there is an open market on Saturdays or Sundays where i can simply drop in to check the quality and price or cars (Instead of calling twenty people and visiting twenty houses).

Location is not a problem.

I really appreciate if someone can give some info.

Thanks,
Indrajit

I think its called Gebrauchtwagenhandel.
Anyone knows something like that in NRW or Hessen