I think once you start mixing up new and 2nd hand sales at the same venue (which is effectively what you'd get if you were to tag it on to the Stalking Show) you lose the essence of what a jumble sale / car boot sale is really all about, and you'd get all the professional car booters turning up with loads of tat to shift, rather than it being a genuine opportunity for stalkers and rural folk to move on their own surplus kit to like-minded folk.