Before buying a second-hand or vintage watch, it would be important to know watchmaking history, especially regarding the technical evolutions that determined the development of certain specific elements in watches, which also helps in their dating. Unfortunately, it is such a vast field that it is practically impossible, even for the most experienced professionals, to be able to acquire a total mastery of all the specific topics, so the best thing, if you are intent on buying a vintage watch, is always to rely on an expert. Your trusted watchmaker will surely be willing to help you in your search for a vintage watch of your interest and will be able to help you avoid falling into the most common pitfalls that a less experienced user could end up in.
A good tip is to rely on "established and professional" sales channels. For example, several e-commerce sites specialize in buying and selling second-hand watches. If they continue to operate for years and have many positive reviews from buyers, then you can be reasonably sure that by purchasing a timepiece from them, you will end up with an authentic watch.
The same can be said for the more prominent auction houses: that is, those that have a curator. In these cases, the auction house entrusts one of its experts with the item for sale so that he can make an assessment about its value and check that it is an authentic and original watch in all its parts. In the case there has been any replacement over the years - and it is possible since we are talking about mechanical objects that can break during use - this fact should be well indicated in the description and expressed in the proposed evaluation. Buying from them also leaves you an official declaration about what you have bought.
A third way to buy original vintage watches is to rely on the power of communities. There are plenty of busy watch forums online, crowded by users who share a passion: in this case, watchmaking. And they populate these forums, which often have marketplaces where users offer their watches for sale or exchange.
If a long-time user of a forum (and usually, it is easy to verify that he is one) offers a timepiece for sale, it is very likely that it is in precisely the condition he says it is. This is because the social pressure of communities is very strong, and this turns out to be an advantage for the buyer. And sometimes, it also allows you to find a new friend.