bay is bad since they take a chunk, I would recommend either using ocuk, hotukdeals or gumtree. Typically try meet in person if possible as that reduces any sort of issues,
Avoid HUKD. Bunch of cheapskates who have no intention of buying anything that they post in response to. Listed my 13" 2011 MBP with 8GB RAM and a 128GB Crucial m4 in truly stunning condition (not a single mark, the keys didn't even show signs of wear!), only to receive all kinds of ridiculous "reasons" stating how it was massively overpriced (it wasn't). Machines that were severely beaten were going for the prices that some were "offering" (plenty of time wasters!), got all kinds of crap such as "oh that machine that sold on here recently had a bigger hard drive" (mechanical, mine had an SSD), "oh it's a buyer's market" (true, but I'm not selling my machine to a scum for £120 less than it is actually worth).
Sold using eBay (but not through eBay, you can figure out what I'm talking about), was sold within six hours at a very fair price on both ends. Of course, don't actually sell through eBay, as it's...eBay.
Stick with forums with a relevant audience, Gumtree, etc.
EDIT: Just realised that I posted in response to a six month old post...