This is a good site for selling second hand good and also purchasing. It is possible to list ads for items wanted aswell.

You can post free ads but if you want to promote your ad it will cost. You can upload 3 free pics as well for your ad.

As with things like this always sell face to face and be careful if you are unsure.

I would recommend as you can find some interesting items on here.

There are jobs pages aswell.

UPDATE: I still enjoy looking at gumtree but be very very careful if you are selling, anyone who asks you to give out your paypal email address because they will give you an extra £30-£50 for postage is scamming you and should be reported immediately.