I don't keep an inventory of anything. I started my layout 30 years ago. When I started I also had a Walthers dealership, as well as a dealership with other distributors. I was able to stock up on items I knew that I would need for my layout and also had some items in stock for customers that were common items. In the 80's when we had a recession, business was down plus I got laid off when the company I worked for downsized and closed the facility I was working at. I had to let the dealerships go and focus on a new job and raising kids.
I plugged ahead with what model railroad supplies I had put aside, but due to having no hobby shops at all in my area, the layout more or less went dormant for quite a few years. When we traveled, I would have a list of items I needed and would search out hobby shops in areas we were going to visit and stock up. When on line retailers started coming on the scene, I slowly started getting back to building the layout.
I am finding many items I didn't even know I had when rummaging through various boxes stored under my layout, some of them being there for well over 20 years. Many of the items are useful, but many I find that I have no use for. I still have a few boxes that I haven't opened. Sort of holding off and some day maybe I'll find something else I can use.
Nope, no inventory at all. Just a model railroad hoard.