I have for sale, a few ITAC systems Mouse-Trak industrial USB trackballs. These are very nice opto-mechanical sensors, and ball bearings. I have used one for a few months and it's pretty much awesome. The PCB is spill resistant.
These are used, and show signs of use. I've picked out the best ones here.
I am selling these on behalf of another GH'er who wants to share the love. I'm only in charge of shipping and fulfulment.
notable features:
USB
ball bearings / steel shaft
Spill-resistant PCB
Omron Microswitches (note not all came with omrons, some have a different microswitch for the middle and right click. I will gladly choose your preference)
Tested and working well
I will likely be modifying mine to use a laser sensor (I'm still picking which of my trackballs to modify this way)
Notice: some scratches on the housing

Look introspectively into the sunset:

Alternate Style

(note: these only have omron switches for the left click)
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I have taken lots of interior photos and can provide them within a few days if wanted.
Price will be $35 + shipping. Please PM me if interested. I will post updates here.
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Note: this is the reference model.
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Interior pictures uploaded for those interested:
http://imgur.com/a/HdaF5