If you want to clear the slave market list through editing the save file, here's how I did it for Wimborn:
1. Open the save file (I used Notepad++ but you might be able to just use plain Wordpad)
2. Ctrl+F "Wimborn" - you should find the text "wimborn":[{"@path":"res://files/scripts/person/person.gd etc etc
3. Delete all/any slaves inside the [] that you want
There will be a }]} at the end of the Wimborn slave market roster; so you can search for that to find the end. On mine, "player":{"@path":"res://files/scripts/person/person.gd","@subpath":"","a etc etc came next.
4. Save the file (I saved as a text file as default in Notepad++)
5. Load the new file in game as usual (it showed up with a .txt on the end of the filename for me).
Edit: If you delete enough/all of them you'll have some brand new slaves at the market.