I'm a retired accountant. I keep an excel spreadsheet inventory detailing my equipment. It includes manufacturer, model, serial numbers, purchase source, date of purchase and of course costs including: sales tax, S&H, upgrades and modifications. I don't tally a cost for the many hours of time spent repairing and restoring old gear, as that is not work but fun.
What is missing, however, is any estimate of current fair market value. I don't care to estimate or know. A print out is available to any visitor interested enough to sit down with me and enjoy listening to some music.