Everything is sold and bought in Marktplaats, from cars to spare pieces for bikes. Its search system is very convenient, allowing you to find what you need (and where and when) quickly. It is also just as easy to post an ad.
I have used it to buy several items and I have never had a bad experience (crossing fingers) :).
In Spain second-hand shops are not very extended so when I found this website it was a great discovery.
It is very popular in the country, having a huge offer. I have the impression that this market is very centralised through this web. Definitely it is wonderfully useful :).
Regarding selling items, another option is to go to a second hand shop. Some of them buy your things and others give you a part of the profit when they sell them.
I have found other second-hand Dutch online shops but I don’t have any experience with them: