I currently have iLife '11 installed on my iMac from a retail disk purchased new from eBay. Under the updates section of the App Store, there is nothing that says iLife Update.
I thought updates for iLife '11 to iLife '13 would be free?
You thought right, the update to new iLife is free.
However, Apple had dropped the "iLife" name a while ago, now the apps are just being called iPhoto, iMovie and GarageBand.
To get the new versions of the apps, follow these steps:
Note, that even pirated and demo versions of Pages, Numbers, Keynote, iMovie, iPhoto and Aperture can be upgraded via App Store. They will appear in Purchases tab a few minutes after the upgrade.