I've never used Gumtree, so can't speak how it all works, but pretty sure there's no buy now facility, people send enquiries and you negotiate either online, by phone or they come to inspect - there'd probably still be a bit of danger of selling it twice, in all reality (eg, say someone from Gumtree enquired, wanted to come and inspect, you're negotiating with them etc, they decide to buy and take it away, then before you can end the eBay listing, it gets sold...)

But, if you have a smartphone (or don't stray too far from a computer) and a quick trigger finger, it'd be a bit easier to minimise that risk.