Garmin's definition of lifetime maps is not exactly lifetime... Their criteria means that if you don't plug your GPS into your computer enough so that you miss two or three consecutive updates out of the five or so released yearly, then you clearly don't use it much any more as far as they're concerned, and the lifetime of your unit has expired.
That's fine by me... I'm a serial updater.
Here's a useful (but old) forum page on that subject though, which says you should be cautious buying a used Garmin with LMT. It may need to be updated outside of using Garmin Express.
And
here's an EBay buying guide on the same subject.