Sourcing movements can be tricky.
You definitely need to know what to look for and it’s always important to ask your seller questions! Ideally, you want to go with a movement that is aesthetically clean, however that won’t guarantee a proper working caliber, as dry jewels/pivots, gummed up old oil, worn gears etc. aren’t always visible. The movement could be from a watch that is 40 years old (or older) and never serviced once in it’s history. With that said, it’s pretty straight forward to advise one to walk away from a movement that’s rusted and corroded. In addition, I’ve seen some movements that, while very clean were missing some very obvious parts (like balance weights). But that kind of stuff is pretty easy to notice right away.
If you are hunting for a particular movement, have a price set in your mind and then factor in the cost to service. Even if a seller says they know the service history and this XYZ movement has been recently overhauled by his independent watchmaker, that is no guarantee that the service was done right (or even at all). Call me a cynic, but I really only trust a movement to be serviced if I’ve sent it up to the Zigmeister myself. His years of 1st hand experience and knowledge are what I trust, and he’s never once given me a reason to think otherwise
Also, just because a movement looks a little rough doesn’t mean it’s one to be counted out either. I’ve had pristine movements and I’ve had some less than so, and some of the less pretty examples ran just fine.
Good luck in your endeavors!