Spring is probably the best time of year to sell a bike on Poobay. Prices vary considerably depending on where you are in the UK though. Here in North Wales trail bikes always sell well and, as Spring gets underway, you often see some real junk going for silly money. XTs are always very popular, I was after one last year but couldn't afford anything. However, if you look at listings from the midlands and the south east they go for considerably less.
It depends how you define 'used' as well

If you do Ebay it you can always withdraw the listing if you don't like the way it's going. It's not very sporting but a lot of people do it.
BikeTrader is another option. I'm not sure what their fees are like now but a polite chat on the phone may get you a few quid knocked off.
Or you could save yourself the trouble and sell it to me for about forty quid

Dave B