Netherlands: where to buy cheap bikes? (Police autions, cheapest towns) We are a french couple visiting the Netherlands (starting tomorrow) by bike. We have few questions: 


are there specific places / city where we have better chances to find cheap bikes ? (except amsterdam, since without bike it will be too big to start with). 
we heard about police bike auction but according to some website it seems reserved to the community or people whose bike was stollen, at least in Amsterdam and Hardenberg (which is understandable). Do you know if they is a website listing all the police auction dates (we can go anywhere in the Netherlands to buy the bikes)? We did try googleing this "Fietsenverkoop bij politie + city-name" (translate with google translate) but we didn't find any info except for the auction in Amsterdam, Hardenberg (where being french resident we can't go), and Groningen (the next auction is too late for us)? Are there other keywords we could use?
what should we do if we find a good bike (not suspiciously cheap and from a private person : we found out about marktplaatz) without any serial number on it (in France there is no identification number on bike so we don't do if it's mandatory in netherlands to have a serial number, or if all the bikes have one). Is there a way to ensure it's not stolen? Would a photo of the ID card + the address of the seller will be enough?
are bikes cheaper in Belgium ?  


Thanks a lot for your help!
There are several places to avoid if you are looking for a cheap bike, and several places to seek.

First to avoid:


Generally bikes tend to be pricier / lower quality in all towns with a big university because demand is high and many bikes are "consumed" by the buyer. Lots of exchange students buy a bike and discard it after a year rather than resell it, and most other students use their bike untill it falls apart with as little maintenance as possible and discard the wreck. So you're less likely to find a good value bike here: Groningen, Enschede, Wageningen, Utrecht, Amsterdam, Nijmegen, Maastricht, Eindhoven, Tilburg, Delft, Leiden.
Regular bicycle shops (they usually charge a high markup for secondhand bikes).


What to seek out:


As a counterpoint to the point about university towns above: in many of these university towns you get people who collect the wrecks and combine them into working bikes that they sell for a nice price. You'll find these people through marktplaats and they offer reasonably priced bikes.
the "Kringloopwinkel" or "tweedehands winkel" in whichever town you select (second hand store). They are quite often the party that receives "left over" bikes from the police and bikes removed for being "in the way" (which I explained in my answer to your previous question)
www.marktplaats.nl (with an "s") is a good source of cheap secondhand bikes. There is no requirement for serial numbers on bikes though insurance companies sometimes require it. You should probably go with your gut feeling when going this route, I think legal sales are the norm there though. You can do two things to improve your odds of staying on the right side of the law:


Check other ads on marktplaats from the same seller, check how long they have been a user on marktplaats longtime users are less likely to be fraudulent.
Dutch police has a tool to find "suspect" sellers. Fill in the sellers' username and/or e-mail address there to check if "reports" have been filed about that user (bikes are in the category "overig").



As to your final question: I think the dutch market for secondhand bikes is very liquid and very large. My gut feeling tells me secondhand bikes will be better value in the Netherlands but I never tried buying a secondhand bike in Belgium.