I sell a lot, all my own stuff as I clear it out, or unwanted gifts. Everything is either new or in great condition. I price it to sell and if it doesn't sell but gets a lot of interest, I will drop the price. Most things are £1-£2, with some things being dearer if a good brand. (I just sold some Joe Browns joggers for £10).
Photos and description are very important. I put on loads of photos, front and back, labels, any details on the outfit. I also describe honestly and put brand, size etc.
The postage is a bit of a killer so people will buy bundles to get the most out of it (and a discount).
I use the money to get DD's Christmas presents. Last year I bought £35 Musical Theatre books for £10, in mint condition.
Sadly I have stopped buying a lot of things on there, due to people selling rubbish, a ripped dirty bag, jewellery with a stone missing. I only buy things that are very well described now.
That is the only downside of buying on there. I think if you can prove that the defect was not described, then the seller should have to pay return postage if they want the item back again.