"M39" describes Leica's screw mount, and most M39 lenses on the used market are Leica M39 compatible. Most. And therein lies the problem.

"M39" only tells you that it's the standard diameter for a metric 39mm screw. That's about as useful as knowing that a bolt is a 1/4" bolt. You also need to know the thread pitch and profile.

A lens that is only specced as "M39" may be a 39mm/26tpi Whitworth Leica-mount lens. Or it may be a 24tpi early Canon lens (which won't fit your Canon 7). Or it may be a 39mm x 1 DIN thread screw (mostly Eastern European lenses), which also won't fit the Canon 7 (or a Leica). Or, rather, they'll fit if you force them a bit, since they're almost 26tpi (25.4 tpi), which may then keep your other lenses from fitting properly unless you force them a bit too. (Rinse and repeat until there are no threads left on the camera body.)

And it doesn't help that most of the later Soviet lenses were Leica mount (to fit the Zenit rangefinder) and that Canon and FED, who were the main non-Leica oddballs in the beginning, went on to become the major Leica-compatible screw mount rangefinder lensmakers. A little research can save a lot of trouble.

And no, eBay isn't your only source. There are still very good used equipment retailers out there, like KEH, that can limit the risks involved in purchasing (especially) the more desirable Cosina Voigtlanders and so on that you might want to consider using on a Micro Four Thirds camera as well as on your rangefinder. And while the prices may not get quite as low as a "really bad for the seller" eBay auction, they're usually better than what you'd get on a "quite good for the seller" auction.