Been doing some measurements for the gear...

The seller claims:

"outside diameter of gear 8.7mm/ screw diameter 6.5mm length 19mm"

right...

Anyways, none of those measurements seemed to match anything in the photo. Finally looked up the specs on this otherwise standard-sized motor (and compared to one I have here) 20mm diameter. Judging by that, the *only* measurement that matches those listed is "screw diameter 6.5mm" which is actually the diameter of the flange. "length 19mm" *might* correspond to the total length of the screw to the body of the motor (including shaft and bearing), but that's more like 17-18mm... So... really not much to go on, here. 20mm diameter on the motor seems pretty standard, so I could try to base all measurements off that. Realistically, I guess I'd have to wait till I get these before I can design around 'em. :/

I think this is the answer: There seem to be several styles...

This page is *great* as it gives specs for one, but actually shows three entirely different styles in various pictures (including the style I based the gear on). This picture seems to show worm-gears that might actually match the measurements they claimed.

http://www.aliexpress.com/item/20pc-6V-4000rpm-12v-8000rpm-Standard-130-motor-Green-Micro-DC-motor-with-encoder/1698459475.html

The next image, again, is the one I based my gear on... different seller altogether, none of their products show the straight-worm-gear (seen above) which probably matches the measurements claimed by the seller showing this picture:

http://www.aliexpress.com/item/Great-10pcs-Speed-motor-MABUCHI-Standard-130-motor-encoder-photoelectric-encoder-DC-motor-high-quality/1903700249.html

Frankly, I think this worm-gear would be easier to mate with a gear (as opposed to, maybe, a linear glider, which the other seems well-suited for), as this has sloped-sides which would work with the rotation of a gear. But, I don't even know if these are the ones they're selling!