First, as far as the C&R goes, it's a license to collect, not a business license. If you let it lapse, you can keep all the guns you bought with it. I believe you must keep your bound record book in case ATF wants to look at it a decade down the line.
As far as NFA items go, a dealer who lets his license lapse can keep the pre-86 dealer samples as his personal property. If he decides to sell them, however, he can only sell them to another dealer; if he dies, his estate cannot inherit them (they must be sold to a dealer). Post-86 dealer samples must be sold to other dealers if you let your license lapse.
As far as C&R NFA items go, you get to keep them but you lose the ability to buy and sell them interstate, and you also can no longer transport them interstate without filing a form 5320 with ATF beforehand.