whats the difference between pre and post ban? and how could you even get caught with a "new" post ban gun? ie: post ban lower and preban upper or vice versa..why would it be worth shelling out the dollers for a pre ban?

The difference is merely a date, & whether the lower was assembled as a complete rifle before that date.  All "new" AR lowers are considered post-ban.  The ban made it illegal for one to possess a weapon w/ more than 2 of the 5 evil features:
-detachable magazine
-protruding pistol grip
-telescoping/folding stock
-flash hider
-bayonet lug
So you see, pre-ban lowers attract a premium because you can legally have all 5 of these features.  If the Telestock isn't a big deal, then you'll be happy w/ the post-ban, as bayonet lugs are (arguably) useless, & muzzle brakes can be utilized instead of flash suppressors.  It all depends on what you want.