In any mouse thread, I will always recommend ploopy trackballs and mice.

https://ploopy.co/

My only gripe about the ploopy mouse is the scroll wheel spins a little too freely.

Do you find the wheel sometimes misses events? I just put together a Poopy thumb, and that's my only real gripe

No, my gripe is that it's a little too sensitive. I keep meaning to look up how to ignore small movements of the wheel in QMK.

Ploopy makes cool-looking stuff. Based on the pictures on their website, it looks like the bodies are 3d-printed? How do they feel to use?

Their nano trackball looks interesting, too... are you meant to bind keys on your keyboard to act as mouse buttons, since the trackball lacks buttons?

My mouse feels just like any other. The 3D-printed nature doesn't really affect how it feels under my hand. I haven't had any problems with dirt either.

I'm assuming you'd bind keys on the keyboard, yeah. I'm a big fan of more buttons on mice and trackballs though: Like a keyboard with QMK can have layers, so can my mouse. Normal side buttons are mapped to page up and page down, if I hold down my rightmost button, those buttons become volume up/down :)

The pictures make them look almost textured / ridged, is that noticeable when you hold it? I went through quite a bit of effort to sand my prototype (it's PLA, can't be acetone-smoothed), and I think it's nicer for it, but maybe the Ploopy's material (ABS?) feels nice enough un-smoothed.

I don't notice the texture at all.

It would probably be much cheaper and faster to use donor mouse for electronics/optics. Good 1000dpi optical mouse goes for $5, if board needs to be shorter it can be safely cut off where buttons are.

I, for one, would buy one of these if the author produced them.

Thank you, I'm looking into options for selling small batches after I make a few more refinements (I'm considering putting a USB-C port in for the cable instead of the current style). Maybe via Tindie or some other platform?

Tindie is a good choice. I think you should create some sort of interest list if you are seriously considering producing these for sale.