I prefer to use Gumtree.
I sold a pair of Genelec monitors on there last weekend. Couldn't have been easier, the guy drove 130 miles to collect them. It may take slightly longer sometimes, though usually things sell within a couple of days. Just be sure to state cash on collection only.
Best of all it's free.
Musical stuff I advertise on here, but that's sometimes a bit hit and miss. It's also rather expensive. My annual selling subscription ran out, I couldn't afford £20 so had to pay £7 for one item. The £20 is reasonable, £7 for a one-off advert is expensive though.
Facebook is a good place to sell too. I sold my Modulus Q6 via Facebook.