On eBay alone, you will discover that at any given moment, there are close to 75,000 pieces of vintage jewelry put up or available for sale. There are numerous sites where you will get more or less the same number of jewelry.
The definition of vintage may vary depending on which site you choose to buy this type of jewelry from. However, as long the vintage jewelry was made between 1910 and 1990, it generally qualifies to be listed as such. If the history of the jewelry in your possession goes back to earlier than 1910, then it is no longer vintage, but now qualifies to be called antique. With that said, you should expect numerous requests to prove the authenticity of your claims that the jewelry you have is vintage. Documentation is important, and you should never lose or misplace the paperwork that proves the jewelry in your possession is vintage.
Most collectors choose to buy vintage jewelry online that were made between 1940 and 1970. Since you will have to put up a photograph of the vintage jewelry online, make sure that the photo is of the best quality. If you are buying, never buy any purportedly vintage jewelry online if the photo appears to be fuzzy. A partial photograph of the jewelry is never to be trusted. If you are dealing with a reputable seller, then he/she will provide clear and unmistaken pictures of the piece of vintage jewelry in their possession. These photos must be taken from different angles to show the proper state in which the jewelry is in, to avoid any doubt.
If you wish to buy the vintage jewelry online, you should be wary of sellers who lack any clear return policy. If the seller does not have a return policy, do not buy the jewelry from them. As long as it is genuine, the seller will have no problem in including a return policy. Before you place any bid for the jewelry in question, make sure that you read all feedback from others. Look for sellers with a stellar reputation, and you will avoid falling victim to a lot of misrepresentation, misinterpretation, and outright forgery that is often the hallmark of this industry. Do what you need to do to protect yourself from losing money on worthless items.
In summary, before you invest in any vintage jewelry online, you need to teach yourself how to look for visual clues to ascertain their age. If you are talking about older brooches, then their backs must be smooth and not patterned. Anyone telling you that they have aurora borealis rhinestones that were made before 1950s, is cheating you, since they only entered the market in the mid 1950s. Copyright symbols started appearing on jewelry made from around 1955, and not any earlier. Most vintage jewelry have signs of wear and tear, especially along the clasp, not unless it was stored away in your grandparent's drawer and nobody ever wore it.