Roughcollie, I also second the advice given, namely purchase from a reputable seller, if possible.
In my own personal experience, here's my suggestions:
1. The Zorki-4 has a bright almost 1:1 viewing window. Easy on the eyes and easy to focus.
2. The Fed 2 is robust and rugged, but the viewing window + rangefinder patch are considerably smaller.
3. The Kiev 4 series of cameras is also a good bet. Especially if you can find one with the "Helios-103" lens, it's a sleeper. The Kiev 4 cameras were built like tanks and seemingly indestructible. I've owned 3 of them, and they've all been good performers.
There is a vendor on the east coast of USA, the website is fedka.com -- he sells only 100% functional items, no duds -- I've dealt with him and had no issues. His prices are higher than eBay but at least his cameras are in good shape.
About "pitfalls" -- well, as everyone else has pointed out, you need to be cautious about where you buy your camera from. Some eBay sellers in Russia, Ukraine, etc. are 100% top-notch, others are not.
Also, it's important to note that the lenses on Russian rangefinder cameras are made to different tolerances than Leica thread-mount lenses (Leica, Canon, Voigtlander, etc.) -- so they may not focus properly on a Leica camera body, especially when wide open. This is known as the "register" against the film plane. The Soviet cameras have a slightly different register. I'm only mentioning this in case you were thinking of trying out a Russian lens on a Leica thread-mount body. It can be done, but you have to play it safe and shoot with the lens stopped down to F5.6 or smaller.
Good luck with your search!