I have a split reaction to your post Danno.
First, you're on the money with one transducer networking through the Lowrance Ethernet to a second display. It's a great feature that allows you complete control of your sonar from either unit.
However, I do have an issue with "used" electronics for a couple of reasons.
First, the best way to kill electronics is to supply them with a weak electrical system. Varying voltage, particularly while starting the main engine will drastically shorten the life of your valuable unit. Since you have no control of the used electronics previous installation or use, you really cannot count on that gear to navigate you home.
In addition, if you do have a failure, Murphy's Law dictates that it will happen on the very best day of fishing, during the peak summer season.
If you've purchased the unit from a local retailer, chances are that you'll get immediate help.
However, your on line purchase? Well, chances are that you're flat put of luck with no product support at all.
This is a case of penny wise and pound foolish. I understand saving a buck but if you buy local, you have help locally.
Hope that helps and good luck!