Some consider old wine that is stored in a dusty basement or cellar, or wine that is left unattended for a long period of time, to be "vintage". But that is usually not the case. Wine is classified as vintage based upon the year the grapes used in making wine were harvested. Some years are known as "good years" for growing grapes, because of the optimal growing conditions prevailing though out those years. Wines made from grapes of these years are usually a cut above those wines made from grapes harvested in other years. Thus one's wine collection may be old, but it may not be good at all.
How can you find the value of vintage wines? You may be able to answer this effectively yourself. If you have some level of proficiency in wine, some free time, and a lower valued collection, you might find interest in the do-it-yourself method. With just a few hours on the internet, you can find some estimated values that are in the ballpark of the actual value of your collection. With this knowledge, you can then negotiate properly with a potential wine buyer.
You may also receive an estimate from specialized wine retailers or wine auctions located in your area at no cost. Just don't take these the estimates they provide as fact. For instance, wine auctions often place exaggerated estimates on collections so that you will sell your wine with them. When the wine sells for less than promised, the auction will get paid, but you will not gain the full value of what you deserve (especially for all the work you have to put into selling wines through auctions). Wine retailers, on the other hand, may work in the opposite direction. They may understate the value of your collection, especially if they believe that you are not comparing their estimates with those of other retailers. Worst of all, they may present a large initial estimate, only to "reveal" troubles with your wine's condition after you've sent them the collection.
One way to avoid this is to hire a certified wine appraiser; one that you, or a friend, may know personally. These appraisers do charge for these services. But if you're an acquaintance or an acquaintance of an acquaintance, you might get a good price. But if there is no such person in your rolodex, then you will just have to pay the price of an appraiser. If your time is money, it may certainly be a better value than spending several days on internet research.