In spite of what was said about database programs, I use Excel to keep inventories of my model trains, model cars, and model ships, and its done everything I want. It's easy to use and I know it well, and it does what I want. And I like to play with numbers, so the fact that I can run many different sort routines, pattern recognition, and other analysis (regression of original cost vs. my rating of satsifaction, or trending of price of locos I've bought over time) that you can't do in a database program (although you can dump from Access into Excel, of course)
The format is up to you: not just what you gather and keep about each, etc.
Thank about what you want and desing you own.