I've heard that lifetime maps does not transfer to a secondhand buyer.
Per Garmin's terms, true.
Terms and Limitations
If you purchase a nüMaps Lifetime subscription (sold separately or bundled together with certain GPS models), you will receive up to 4 map data updates per year, when and as such updates are made available on Garmin’s website, for 1 compatible Garmin product until your product’s useful life expires or Garmin no longer receives map data from its third party supplier, whichever is shorter.
The updates you receive under the subscription will be updates to the same geographic map data originally included with your Garmin product when originally purchased (it is possible to use nüMaps Lifetime if you purchased a City Navigator map on DVD and it is used on the same computer you installed the City Navigator maps). In some instances, your Garmin product may not have sufficient memory remaining for you to load an update to the same map data originally included with your Garmin product, in which case you will need to either (a) select reduced map data coverage for your updates, or (b) purchase separately a microSD/SD card (as applicable to your Garmin product) and load all or a portion of the map data coverage for your updates to the card and insert the card into the microSD/SD card slot contained in your Garmin product.
Garmin may terminate your nüMaps Lifetime subscription at any time if you violate any of the terms of this agreement. Your nüMaps Lifetime subscription may not be transferred to another person or another Garmin product.
Basically, when you connect to Garmin with a lifetime maps gps unit, it will detect that you are not the person who registered the unit and not update maps.
Honestly, how does Garmin or the GPS know you're not the person who registered it? Does it require a retina scan/finger print verification/urine sample before it will turn on and update?
As long as you have the original owners registered info to get map updates, you'll be fine.
Or you can use the work-around
But I'm wondering if Garmin has a way of shutting off lifetime traffic to secondhand owners?
Let's say I registered GPS unit. I then upgrade to a new unit 6 months later and give old unit to wife/son/daughter/father/family member/friend to use. You're telling me Garmin's going to prevent them from using the traffic service/device I paid for? And how is Garmin going to know whether device was sold or was given away for free?
You just plug in GPS, turn it on, wait for it to communicate with satellites/receive signal. That's it.