Hello jim,
for my uroMAMEstyx project,
I've ordered most of my parts from Terry.
All the transactions were smooth and shipped rapidly.
I can only recommend him!!!
I have a yellow translucent trackball (USB/PS2) which support up to 3 mouse buttons.
You just plug it in and you have a working mouse. It's as simple as that! you don't need an optical interface of keyboard encoder with trackball support.
As for your questions:
Are these NOT the same item? Why such a big difference in price?
Seems to be exactly the same thing

It has the happ sticker on it.
Why such a big difference in price?
I don't know why such a big difference, but you can rephrase your question :
"why pay more?"

If you want to see the pic of the unlit trackball, you can go to my project thread uroMAMEstyx in my signature.
PS:
if your keyboard encoder already support a trackball, you can go with a regular trackball (no usb) which are easy to get in the 60-65$ range... In my case, I bought the GGG Keywiz and got the usb trackball.
Have fun with your project!
Jay