Maybe against my better judgement but I thought I'd share my experience using the Play.com second hand service as a seller.
It's bloody brilliant.
I've managed to shift games I would never be able to move in the swapsies thread here or would be worth nothing trading them into Game or the like and made a very healthy amount of cash doing. Easy to set up and you get into it straight away. Links to orders and stuff that you need to process aren't particularly obvious but you get a good set of tools, including being able to issue your own refunds.
Items stay listed (for free) for 60 days. Play.com charge 10% +50p on your transactions (so if you are selling a console I wouldn't bother) and the cash goes into a Play.com balance. You can have the money transferred to a bank account for a further 5% fee.
To give you an idea of what sort of prices I was getting I'll list what I have sold so far and for how much:
Dynasty Warriors 6 Empires: £11
UFC 2010: £7.50
Spiderman Shattered Dimension: £15
Red Dead: £14
Dead Space: £9.99
Samurai Warriors 2: £10 (this actually cost me 2.99 from GAME)
Halo Reach: £14.50
Not a bad little haul. It cost me 1.50 for three bubble envolpes and £1.09 each to post. I think UFC 2010 is just about the only one where I would have got the same in store for my costs.
So I'd recommend it. People can be divvies and rarely leave you feedback, but especially if you are willing to let your funds sit in Play.coms balance, you can do pretty well out of it
My own slice of the internet is here: http://www.maulsquad.com - it isn't very good.