Caution: Information about places to buy changes quickly! This article was written in January 2017, and reflects the situation and my opinion of that moment.
For solid state materials, the NFOR has a partner forum, the Nederlands Transistorforum, with roughly the same culture and rules (and the same people). When posting, be aware that the separation between tubed and solid state discussions is maintained quite strictly!
Principle language on both Forums is Dutch, but you can post in other languages (posting requires registration). German, English and French will be answered swiftly. Posting in other languages, though maybe not prohibited, is useles because nobody can read it and you'll get no answer. Most of the frequent sellers on NFOR and NTF have their city listed, so you can select on offers nearby.
Nice about Marktplaats is that you can enter your location, and search within a desired range (like 5 or 25km). Not so nice is that the site has become quite commercial, and floods you with paid advertisements.
Some sellers operate on semi-commercial basis and
offer top goods for top prices,
but you also find worn out stuff for low prices.
And, of course, worn out stuff for top prices,
but you can ignore that.
And incidentally, cheap a golden find.
For example, I bought
this Philips B1X42A,
well repairable, for €12.50, which I consider a nice buy.
Despite the common label Kringloop, the shops differ quite a lot in what they offer. They mostly provide used household articles (furniture, books). Usually there is a section that carries electrical equipment, including audio stuff. But nice radios have become quite rare, although one can always be lucky here. For spare parts and bastling material it is a good place: speaker boxes, PC parts, etc., are usually found at low prices.
In Zeist there are Kringloopwinkel
in the Jan Postlaan,
Kringloopwinkel De Markt in de Voorheuvel,
and Het Goede Doel in the Johan van Oldebarneveldlaan.
Unfortunately, sufficient volunteers could not be found in 2016,
and the Activiteitenmarkt didn't take place.
The future of this activity is unclear.
To find Rommelmarkts close to you, you can search (by province) on Meuk is Leuk. On King's Day (April 27, King Willem Alexander's Birthday) there are King's Day's Markets in almost every city and village.
In Czechia there are many, in Slovakia there are less.