I have no knowledge of this particular vendor BUT, after being burnt myself, would urge anyone who buys through Gumtree to exercise caution before parting with money. We have got used to marketplaces like eBay where there is some measure of buyer protection, but there appears to be no recourse through Gumtree - so I would only either hand over money in person when collecting the item, or pay via a mechanism which allows you to recover your money if the item doesn't show up.
And if it looks too good to be true, it probably is !
Dave