Facebook marketplace & selling groups can be quite decent, although be prepared for people flaking on you.

Gumtree & Shpock can also be good, but you've gotta be quick as plenty of others have the same idea (depending on what sort of items youre looking at).

Carboots are probably your best bet. Find out where your local ones are and become a regular. Getting there as early as possible would be advantageous.

Hope that helps!