What is use of those SN? why someone would pay 5 bucks just a serial number of someone else's monitor??
To issue so-called warranty fraud and get a replacement. Jabra, Logitech, and few other companies are extremely easy to do it.
I would suggest anyone NOT purchase SNs for any Logitech stuff; their way of creating S/N was recently cracked and there's high chance he's not selling stuff from products but generated ones.