First do pip install --user -r requirements.txt (or use a virtual env or something, idc). Then do flask run. That should be about it.

Grab the API call from Inspect Element → Network that starts with https://www.marktplaats.nl/lrp/api/search? and stick it in the input box.

If Marktplaats gets mad at you for using this for some reason, don't get mad at me. Use at your own risk.