There is nearly zero reliability in boot sizes. Even mondo sizes, that are supposed to be the metric measurement, aren't reliable. Adding in that you want used boots makes it even worse because different feet will break in boots in different ways.
It would be awesome if companies started making PDF files that have the outline for the stock footbed but that would take away from in store sales.
That might help with shape of the footprint, but doesn't include elevations, arch support, and other things that contribute to good boot fit. Sadly, the best and arguably only way to get boots is to try them on yourself, preferably with a qualified boot fitter helping you out.