I love how people suggested we should be running a dual tracking system. I'm just curious, not for tax purposes, but this site was how I was tracking what sets I owned and how many of them I had if more than one. I have since moved to an excel spreadsheet and inventoried a bunch of rubber maids and boxes full of stuff; but I did have all my dates and purchases prices stored here. I don't have receipts lying around from 10 years ago except for a few types of items like my homes, automobiles and jewelry.
I'm a selective collector not an investor, the site attracted me because I could quickly update purchases. Plus I actually found sets from my childhood. Remember when firemen had multiple swivels on their arms and you built there body from 2x2 blocks? I found that and a few others in the attic. The site was useful to show me that I was missing a lot of pieces if I wanted to make it complete.