You log in and click on "Add collection" for any specific item - then fill in all your details, eg. how much you paid for it and when, and any other notes you may want to add for it - then whenever you're browsing, you'll see a tick next to whatever's in your collection (that you've already added).
Da da da da....then, once you've loaded a few, you can click (around top right) onto "my own" to see your list and total value of your collection. You can make it private or public.
I think some of the values on the site are wrong or too low, but that doesn't matter, since you can put in your own estimates for your own items. The great thing about this site though is that is seems to have virtually EVERY Madonna release ever throughout the world (at least it's the most comprehensive I've ever seen I think).