Pro#s and cons for each selling site (IME):
E-bay: very flexible, you can auction, buy it now, or best offer (sort of like classifieds). Local people can pay cash on collection. You have back-up if they pay by PayPal but you have to sacrifice in terms of commission and fees. If posting, make sure you get it weighed or get a quote first, otherwise it may cost more than you have advertise for P&P. Take photos of how you package it, as I've experienced royal mail/couriers who destroy stuff and then
eBay usually side with the buyer and force you to refund money. Taking this up with the carrier is a nightmare, as technically once paid for it's the buyer's problem but Ebay do not care about that.
Gumtree - totally free and simple, cash on collection, but be prepared to be messed about by people not showing up when they say. Shouldn't be an issue with an expensive bike frame though, it's more for things like cheap cars/bikes/exchange and mart stuff. You can't quibble with cash in hand.
Facebook - same as Gumtree but becoming a really popular way if selling. It has the advantage of making more people look at it whilst on FB, rather than requiring the buyer to actively go onto Gumtree.