Swapfiets builds their own bikes and does not - never ever - sell them. If one gets stolen, it can‘t be resold without one noticing it’s stolen, as these are not in circulation in the first place and still owned by the company that built them

The same logic as with U-Haul trailers [0] - if they don't sell them then there's only one logical explanation when a random one shows up.

[0] https://www.quora.com/Where-do-I-buy-new-U-Haul-trailers/ans...