I never really used eBay until about 6 months ago. I had an account but hadn't put it to any use. I was at work and my boss was telling me about how much money she had made just from selling things she didn't want - she was even planning on going on holiday using her funds! I decided to log on and take a look at how much things were going for that I had lying around. Everything from clothes, bags, shoes, electrical were being sold. I had plenty I would have just thrown away but instead I made myself some money, which was a huge helping hand especially with Christmas fast approaching.
You do find popular highstreet brand clothes such as Topshop and Riverisland go for alot more money than first thought whereas the lesser heard of shops only tend to make you a pound or two. I even sold fancy dress outfits that I had stored in a box under my bed for 10-20 pounds each! When you have been selling for a while you tend to recognise what at the more popular items.
I've made over £200 pounds just having a clear out. You would be suprised what people will buy from you for a discounted price.
You could even pick up some fantasitic bargains as a buyer, I have saved an awful lot of money buying from eBay rather than highstreet shops.

A couple of tips I would give someone interested in selling are:

When setting up my account it was alittle bit confusing, I did stuggle to navigate to where I wanted to go and set up paypal but once it is done its very easy. If you are planning on setting up an account I would reccomend that you ask someone with experience to help you and show you the ropes.

When listing items for sale be realistic with your postage prices otherwise you won't get much interest. You learn as you go with how much things will cost but the Royal Mail website has a postage cost finder.

Be honest, if their are slight marks, scratches etc on items then make sure you advise the user. If you don't they can complain and leave you negative feedback.

Post items promptly and make sure you keep the receipt just incase items go missing.