protocol is pretty simple, don't ship anything unless you've got the buyer's payment in hand (or with trusted escrow if we're talking very high dollar)
as for is it OK to ship, it completely depends on the item itself being shipped, and to where... (I would not ship outside of the lower 48, but that's just my pref)
I would ship anything that was shipped to me.
I would NOT ship for example a 72" span kit built airplane with a one piece wing. (between the wingspan and the size of the fuselage including non removable empanage, I'd need a HUGE box that could not be crushed... (which isn't likely))
if it were a kit that hasn't yet been assembled, then no problem, ship away.