1989 Dom Perignon Champagne just bought at the store, How much would it retail for if resold?

You may have taken a risk here, the bottle standing up right and also exposed to sun light. Most older bottle of wines and Champagne are well taken care of, stored properly in a case or warehouse that is Temperature controlled. So I would have to say you took a chance and if you sell it to someone they may ask these questions. So what to do, well give it a shot and drink it for your party. If you get a POP from it and the color doesn't look off, you maybe in luck. I hope it works out for you and you enjoy a great bottle of champagne.