used night vision?

Is used gen 2 or 3 night vision worth the cheaper price? Does gen 2 or gen 3 get significantly less effective over time that buying it used would be that much cheaper?

It would depend on how used. All conventional night vision has a finite lifespan. Intensifier tubes last only so many hours. However, there is a significant improvement in lifespan in Gen 3, compared to Gen 2. Going used can save you money, yes, but unless you know how many hours have been run on the unit and unless you know the overall condition of the unit in terms of other features, you are buying a pig in a poke with no warranty. Buyer beware, as always.