What component do I need to add my own "Private" electric meter
Yes, you can do that and as you have noted, there are many used KWH meters for sale on EBAY and other places.  A properly installed meter base along with a listed meter will do what you want and be compliant.  It's not uncommon for such meters to be used with solar energy systems and such.
I recommend the old-fashioned mechanical kind vs. the new digital models.  They are simpler and more reliable in my opinion plus they look cool.  I like the spinning disk vs. a tiny LCD &quot;spinning&quot; one way or the other.