"Could some knowledgeable members give tips on what is important? What questions should I ask?"
You have the basics covered...IMO
The questions we asked were...
How long allowed in park, and how much time out before you can use enter their system again, and how long can you make reservations in the future. Yes, lots of "in's and outs" and a new language. Have met many that full time going park to park, if that's what you want to do.
"What are the best memberships?"
Depends on your opinion of what "best" is.
Some people spend a fortune and the "best" to them is as many parks as possible. Some people (like us) have a very old Thousand Trails and RPI but that's all we need and happy. I guess the "zone" type membership would work for people that don't travel much.
BTW we are not very happy with TT renting out spaces long term to people that are not there camping, just sucking up space IMO.
I've heard of people calling TT and giving the membership number they are thinking about buying and asking TT what that membership can and can't do.
Personally I'd buy from on of the links you posted and not Ebay.
We very seldom use reservations.
2003 Majestic 23P... Northwest travel machine
2013 Arctic Fox 25W... Wife "doll house" for longer snowbird trips
2001 "The Mighty Dodge"... tow vehicle for "doll house"