Buying second hand devices is a glamble, no matter where you are. Give ebay.com.uk a try and make sure you're buying from old, trusted, renowned accounts.
If you don't mind being on a contract, check carphone warehouse's website, they usually have some very good deals there (free device+contract+BIS, playbook+free device+contract+BIS, etc).
You can try gumtree.com, similar to ebay. I temporary posted my BB on it aand i got a quick response. Besides, you can buy used BB's from next to nothing in the UK, street value.
