I just sold a MacBookPro to a Canadian friend, and there are some "interesting" issues with the buyer unable to update any apps that I installed (like iLife) and updated using App Store updates, since App Store apps are not transferrable, per Apple. And with all updates coming now (in Mountain Lion) this way, this is really causing frustration. She had to delete my apps, and re-buy them, starting with iPhoto.
Apple needs to fix this! The process of buying and selling a computer has really gotten wonky!
"Not everything that can be counted counts, and not everything that counts can be counted." Albert Einstein