After a collectable has been purchased, its retail price no longer applies and its value is linked to what is called the secondary market. There is no secondary market for an item unless someone is willing to buy it, and an object's value is whatever the buyer is willing to pay. Depending on age, condition, supply, demand, and other factors, individuals, auctioneers, and secondary retailers may sell a collectable for either more or less than what they originally paid for it. Special or limited edition collectables are created with the goal of increasing demand and value of an item due to its rarity. A price guide is a resource such as a book or website that lists typical selling prices. Products often become more valuable with age. The term antique generally refers to manufactured items made over 100 years ago, although in some fields, such as antique cars, the time frame is less stringent. For antique furniture, the limit has traditionally been set in the 1830s. Collectors and dealers may use the word vintage to describe older collectables that are too young to be called antiques, including Art Deco and Art Nouveau items, Carnival and Depression glass, etc. Items which were once everyday objects but may now be collectable, as almost all examples produced have been destroyed or discarded, are called ephemera. Psychological aspects Psychological factors can play a role in both the motivation for keeping a collection and the impact it has on the collector's life. These factors can be positive or negative. The hobby of collecting often goes hand-in-hand with an interest in the objects collected and what they represent, for example collecting postcards may reflect an interest in different places and cultures. For this reason, collecting can have educational benefits, and some collectors even become experts in their field. Maintaining a collection can be a relaxing activity that counteracts the stress of life, while providing a purposeful pursuit which prevents boredom. The hobby can lead to social connections between people with similar interests and the development of new friendships. It has also been shown to be particularly common among academics. Collecting for most people is a choice, but for some it can be a compulsion, sharing characteristics with obsessive hoarding. When collecting is passed between generations, it might sometimes be that children have inherited symptoms of obsessive–compulsive disorder. Collecting can sometimes reflect a fear of scarcity, or