This is a quote from a discussion on the 'Hobbytalk' forum...just the poster's recollection, it seems, but apparently well-informed:
"Monogram and Aurora were the first to wrap kits for security against pilferage and parts loss--they both used Cellophane, machine wrapped (in the pattern of gift wrap) until about 1964-65, when they went to a polywrap material, still machine wrapped and sealed. AMT, MPC, JoHan, Revell and most others didn't take this up until pretty much the advent of the modern shrinkwrap machinery in the late-60's early 70's, preferring to use various tape or label seals on the sides, or sometimes even the end-panels of model kits."
As a 10-year-old in 1966...having built models since around the age of 6...this roughly accords with my memories. The early Aurora jets I had had no shrinkwrap; but by the time the 'Batman' (and superhero/monster) kits were coming out from 1966 on, the shrinkwrap seemed to be common.
Still, I view 'factory sealed' kits on auction sites with skepticism; most I have ever purchased...some actually with cut-marks on sprues and paint smears on the instructions...were poly-wrapped instead of the cellophane I so-well remember. And as I understand it, most collectors prefer unwrapped kits...figuring that being able to make sure kits are complete -- obviously not an option on an auction site, I realize -- is more important than an alleged 'factory seal.'
Like the rest of modern life, it's a matter of balancing risk v. reward. Always a challenge. 
Good luck.