I dropped in last week and it appears you can jump in again when they are first started. They used to be quite strict about not letting you in them.
It has to be said that the vendors do have a point, I remember being at BCA Enfield on a Saturday morning when a bunch of young guys were "red lining" every BMW there, if they were my cars I'd be mightly pissed off...?
I also know one particularly trader that, on a saturday when the auction is full of mostly private punters, deliberately 'crunches' the gears when trying the clutch to make the cars appear to have a problem