Depends on what item you are looking for!
NCT Sales apparently have awful queues and its mad, so that may not be an option.
Freecycle is obviously fab, I got my moses basket off freecycle :), but I've also had the experience of picking up a travel cot that was disgusting and looked like rats had cut through it and made holes, it was awful. But don't worry, most people don't do that!
Gumtree is good as you get to see the items before you buy. So that's good.
With Ebay I've had two experiences with clothes, one was great and the other is a complete nightmare (clothes were nothing like described). I would not buy second hand clothes on ebay anymore due to that, but I would consider buying furniture. But where I live, very little comes up in my area for furniture, as its a small city.
I think its best to buy things when you can actually see them. Unless of course the bargain is too good. I also found with Gumtree that some things are not worth buying second hand, as you can add a couple of quid, and get it for new with discounts/sales/code/cashback. I therefore decided to buy my cot instead and got it for less than £50 from mother care for new!