I've had this experience on gumtree few times. I call the guy - I'll pay the full price, when can I pick it up? (Usually really good bargains) and the seller says - yes, great - time and date and we're set.
Hour later - Hi Pawel, I have received quite a few replies, so I thought the best thing to do is an "auction". If you can send me your offer by tomorrow midday and it's the highest - the item is yours. I hope you can understand.
I do understand... but why didn't you take it off gumtree if we have already agreed on the sale?
Nothing you can do about it...