Years ago in the mid 1980's I started a list of Nickel Plate Road items available, including road number, loco or car type, painted or not, prototype or not, manufacturer, year produced, price, plastic, brass or wood, etc. That list has grown like topsy and is currently available for my fellow NKPers and others at the NKPHTS web site (NKPHTS in your browser and you'll get there and then look for the list).
I started using 3X5 cards to keep track of my brass models and that grew to a windows spread sheet after using a manual listing spread sheet. It also allowed me to make several presentations at train shows in several states and Ontario, Canada. I call the clinic or workshop "Keeping Track," and it has been fairly well received. The best reason for keeping track of your models through an inventory is in case of theft and for your estate planning. It also provides a means to avoid duplicate purchases.
Bill