what's going on is that Nationwide Access LLC, the parent company of Gasbike.net, is the ONLY legal importer of chinese bicycle engines, and the grubee skyhawk factory is the ONLY factory in china that's been certified by the EPA to ship engines to the US.
this means that every motor you see is either an earlier model brought in before 2010, smuggled in from somewhere else (canada, most likely,) or the grubee skyhawk GT5 series.
ALL vendors have to buy new engines from Gasbike, unless they're getting them illegally, or from someone with an overstock of older motors, or if they themselves have a stock of older motors.
so, the supply depends on how many Gasbike orders at a given time. if they buy one container load, those must be meted out to various suppliers, i'd assume to the highest bidder, or to favored customers. after that container load is sold, everyone must wait for another shipment.
if you notice, everyone seems to suddenly have tons of motors in stock, then they all run out at the same time, then they're all in stock again. and some vendors don't even bother to change their websites, and will send you whatever motor they have in stock, regardless of what they advertise.
it seems the supply isn't meeting the demand, and it's possible that the vendors are as jumpy as everyone else, and think if our evil empire sneaks a ban on 2 strokes in overnight, they'll be stuck with un-sellable inventory.
that's the jist of it anyway. i may not be 100% accurate with every statement, but that's about as close to the truth as anyone's gonna get.