I also use an Excel spreadsheet. I enter the name, system, condition, date of purchase, purchase price, shipping costs, customs, total price and special features and have a hyperlink to an offline version of the Ebay website and photos.
Probably typical German
I have almost similar excel spreadsheet like ichich, but I also try to collect and write down some specific information for every box/item I have. I`m interested to know who were the previous owners of the box/item (if possible) and how, when and where the product was originally on sale etc...
For example I bought a while ago two Thimbleweed Park Limited Run Vinyl versions and they were sold out in couple of days so I add that kind of information to my bookkeeping too