Do the 2GIG-CO3-345 units determine end of life by the date stamped on them or 5 years from activation? In other words, if these have been sitting on a shelf unused, does the 5 years start when activated or do they go by their expiration date regardless if they have been activated or not?
The end of life date on life safety devices is based on the manufacture date, not when they are first installed. You should always replace the device if it is past its end of life date.
You should never purchase used Smoke or CO detectors for use with your system, and especially with those devices make sure to purchase from a reputable seller.
Unfortunately it is impossible for a device to go from Manufacturing to Retail immediately, and the products may be manufactured in a staggered fashion, so they will typically be at least a few months old or so.