may seem a odd question but do the NMRA and other large organizational rolling stock (tankers, boxcars, hoppers etc) are these collectable? I mean I bought 3 of them from ebay for good prices and was just wondering… should I leave them “AS IS” cause they become collectable or can I go ahead and strip them down for my own freelance railroad?? I have one caboose from 1990 … a boxcar from wash. 1982 convention and a roundhouse tank from 1983…
I’ve heard of people collecting them, but I have no idea as to values. I’d keep them as-is if they were mine 
Old toy trains like Lionel, American Flyer, etc and modern products in these lines have a collectors market. Other manufacturers and scales don’t have any established collectors market. So while there are HO collectors and N collectors, etc. you’re not going to find a lot of demand, hence not a lot of collector value. If you got them cheap on ebay I’d say go ahead and strip them, modify them etc. This is a hobby not an investment - invest in the stock market.
Enjoy
Paul