Louise,
Ask at the auction if the 'Mantis' is a runner and if so get them to start it up and run it for a few minutes, To Start, press the switch in on the handle give the primer a few tickles pull out the choke and give the starter a few puls, if it is good nick it should kick up with no trouble, also check if there is any other equipment with it some of them come with all kinds of kit, if you can't see it running I would leave it alone :)