You can keep a record of games you own on the site and easily modify it as needed, all from your site blog. Simply access your blog while logged into your account and click the "Games" link on the bar below your header image. You will bring up a page where you should see a box with some text and a user option: "If you need to add new listings for a console not currently included on your list, click here to select the system and start adding games."
Click the hyperlink provided in that box to bring up a list of systems currently featured on the site. Then click the desired system to see a list of games in the database for the corresponding system. You can then check boxes next to the ones you wish to add to your list. From left to right, the boxes indicate that you own the game itself, the box, and the inserts. The fourth box in the row lets you specify that you own a digital copy of the game. Check only the boxes that apply for each title, then click "Update my list..." to make the desired change. Now you can view your list and click "edit" next to a system's name if you need to make updates.
While using the above method makes it easy to get your list started while adding a bunch of titles at once, you might only wish to add or subtract a single title at a time. You can conveniently do so from the main site. Simply access the game profile page for the desired game, where you will see options to add a physical or digital copy of the game to your collection (if you don't already have the game marked as part of your collection) or to remove it. When you click a link to add a game, you'll add only the game itself. You will then be presented with options to add the manual and packaging, or one or the other.
Play around with the feature for a minute and you should be able to use it easily in no time. Maintain your list regularly and you can easily take it with you the next time you go shopping at your favorite retro game store. If you own console, handheld or mobile games that aren't yet listed and you would like to catalog them, you can post in this thread to ask site staff to add them to the database.