For old books, Cd's and DVD's valued at over £1, I use Amazon.
For old CDs and DVDs worth less than this, I use Music Magpie.
For anything else, I use ebay.
As a newbie to online selling I've decided to give it a couple of months before ellaborating on each of there revenue streams.
I also feel selling stuff online is a little different to generating an online income as it's not the same principle as making something from nothing (other than the products of hard work and time), it relies on you having an item to sell first. However, on the understanding that everyone has a bit of junk accumulated somewhere at home, and that one man's trash is another man's treasure, I suppose there is an argument that this is a viable online income source for anyone.
Also, there may be a scope to buy items cheaply using a site like 'hot deals' or securing a good amount of cashback on them, and then selling for a higher price on ebay. Again, a territory which requires further research and expertise, for now I will concentrate on selling a few old items of my own, and learning my way around the basics of online retail.