There are a few ways you can purchase a watch. I will go through the main avenues people usually buy watches from and some of the risks you may run into while purchasing the watch.
When purchasing through an authorised dealer, out of all the methods mentioned this is the safest way you can purchase the watch. You will not have to worry about the authenticity of the watch. However, the only downside would be the price of the watch. Finding certain deals on watches will not be that prevalent through authorised dealers, which is why usually people search and go through online stores for watches. I would recommend going to an authorised dealer if you are purchasing high-end watches.
Ebay has one of the highest risks when it comes to purchasing watches. There is a huge risk that the watch sold might have some defects or even have fake movements or is just a pure fake of the watch. When purchasing a watch through Ebay, please do ask as many questions as necessary to the seller to make sure the watch is free of any physical defects and is an original watch. A good way to see if a watch is original is by asking a picture of the movement, and then cross-checking between an original picture of the watch through the manufacturer’s website. Also remember to ask or check if the watch comes with the original box or papers, and if it doesn’t then make sure the watch by itself is authentic. If the price is not worth it for a watch without the original box or papers I would avoid purchasing this watch as a general rule. There has been a lot of horror stories particularly from Ebay about people purchasing watches and them turning out to be fake, as well as having a lot of defects not seen through pictures. Some of these people have not even been able to get their money back for the fake watch they have bought.
Amazon on the other hand seems to be a bit more reliable when it comes to purchasing watches, but that does not mean it does not come with its own risk. Still remain vigilant when purchasing watches, and ensure when you receive the watch it comes with all the original packaging as it is supposed to. Amazon seems to be a bit more buyer friendly when trying to resolve an issue with a fake watch and getting a refund.
There is one subreddit you can go to if you want to purchase watches which is r/Watchexchange. This is where people usually sell their second-hand watches usually at a good discount. People here generally have to follow the rules which means to post an album of the watch. Also, if you want you can message the seller directly and ask extra questions, pictures and even videos, and if they refuse you can simply just move on and go and find another watch to buy. They also have their own seller feedback system, which shows if the seller has had previous transactions. A generally high transaction count usually means they are a trusted seller. It is worth going through this for a quick look as you might be able to get your dream watch at a great discount.
Chrono24 is unique in the sense as it allows for both private sellers and independent stores to sell their watches online through their own secure-checkout procedure. They hold the money in an escrow account, and only when you get the watch physically in your hands and confirm it will the money be released from the escrow account to the seller. You can take this time to check the authenticity of the watch as well as whether this is exactly how the watch was described. There is also a chat feature where you can message the seller and ask any additional questions you have and also be able to negotiate prices if need be. I would say the one downside I can think of, is that some sellers do have a bad reputation of not communicating back to their customers. This usually can be indicated through their feedback and reviews. Always remember to ask questions before purchasing the watch, such as the authenticity, the papers, the box as well as the picture of the movement if not provided.
There are plenty of other websites and platforms where you can purchase a watch, as long as you are careful and wary of tricks that some people use online. You should generally be fine purchasing a watch, however if it is a very high end watch, I would go to an authorised dealer. If you are confident about the authenticity of the watch and you know what you are looking for, buying a watch online will usually be pretty safe.