The way people decorate their homes at Christmas tends to fall into one of two camps. Either they buy a new tree and a completely fresh set of decorations every year, or they've been using the same set which is kept in storage 11 months of the year for as long as they can remember. The family who buys new gear every year may feel like they're going better, but the family who recycles may be doing better in the long run - because their well-worn ornaments may now be priceless antiques.
Blown glass ornaments from the late 19th century are worth the most of all - frequently over $1000 each - but vintage sleds from the 1940 and 1950s are worth several hundred dollars now. If your grandparents' tree and the ornaments on it look impossibly old, maybe take a closer look at what's hanging there.