why? Iv sold literally dozens of bikes on gumtree over the last year or so and the only trouble Iv had is a couple of knobs arranging to view and not turning up, as far as I'm concerned that's the best place to sell as there's no fees whatsoever and as the seller you have complete control, if anything goes wrong with the sale I.E. accepting payment by cheque or online payments etc that bounce or something then it's no ones fault but your own. Only accept payment in cash and check for fake notes or bank transfer and make sure it's cleared (actually phone your bank) and your home and dry.