I buy and sell a lot of bikes, personally I bite the bullet and sell on eBay and pay the extortionately high fees. The simple reason being in my experience if I advertise on gumtree/shpock etc then 9 out of 10 people are timewasters/bellends and dreamers where as on eBay iv found that "most" people that make contact are genuinely intrested in buying. I suspect the reason for this is on gumtree/shpock there is no way of seeing who the buyer is so the timewasters/bellends feel safe in playing silly buggers whereas with eBay once the auctions ended the seller gets the buyers name address and phone number.
The other option depending on the bikes value is getting a local bike dealer to sell it via their shop for a set fee, this is handy for high value bikes as bike dealers are set up to offer finance on the bike whereas you won't be.
