In Finland there is basically 2 big sites. Where most units are being sold and even rented.

https://oikotie.fi (159€ for private seller)

https://www.etuovi.com/ (149€ for private seller)

In addition each agency shows the available units on their sites. Then there are some in newspapers and I think there is some papers from some group of sellers.

Ofc, there are some private sellers on Facebook, and some local craigslist equivalents.