People are all the time writing to me about this or that old bottle they've found, usually in the liquor cabinet of a recently-deceased relative. Whether they ask directly or dance around the question, what they all want to know is, "is it worth anything?"
The short answer is "probably not."
The slightly longer answer is:
It is illegal to sell alcohol without a license.
Because of the underground nature of spirits reselling, accurate predictions of a given bottle's value are impossible to make. Appraisers need a record of recent sales of similar items and no such record exists for most alcohol products.
Most old bottles are worth nothing.
Even bottles that may be worth something are, at best, worth a couple hundred dollars. You won't send your kids to college by selling the contents of grandpa's liquor cabinet.
The only significant marketplace is eBay.
Straight spirits such as whiskey, brandy, vodka and rum are usually okay to drink if they've been in a well-sealed bottle, regardless of how old they are.
Low proof products (less than 40% alcohol) can go bad and should be discarded if more that a couple years old.