and for Great Britain and Commonwealth stamps most collectors use Stanley Gibbons catalogues. Typically stamps will sell at auction for a range of 10 to 30% of catalogue list price, but if the stamp is of the very top quality then the sell price may exceed the catalogue list price. As with many collectibles with no inherent value, sell pricing is set by the market bidding on the item at the time. Many collectors also ask their family and friends to save stamps for them from their mail. Although the stamps received by major businesses and those kept by elderly relatives may be of international and historical interest, the stamps received from family members are often of the definitive sort. Definitives seem mundane but, considering their variety of colours, watermarks, paper differences, perforations and printing errors, they can fill many pages in a collection. Introducing either variety or specific focus to a collection can require the purchasing of stamps, either from a dealer or online. Online stamp collector clubs often contain a platform for buying/selling and trading. Large numbers of relatively recent stamps, often still attached to fragments or envelopes, may be obtained cheaply and easily. Rare and old stamps can also be obtained, but these can be very expensive. Duplicate stamps are those a collector already has and are not required, therefore, to fill a gap in a collection. Duplicate stamps can be sold or traded, so they are an important medium of exchange among collectors. Many dealers sell stamps through the Internet while others have neighborhood shops which are among the best resources for beginning and intermediate collectors. Some dealers also jointly set up week-end stamp markets called "bourses" that move around a region from week to week. One of the most well known bourses is the Carré Marigny in Paris. They also meet collectors at regional exhibitions and stamp shows. Collecting specialties A worldwide collection would be enormous, running to thousands of volumes, and would be incredibly expensive to acquire. Many consider that Count Philipp von Ferrary's collection at the beginning of the 20th century was the most complete ever formed. Many collectors limit their collecting to particular countries, certain time periods or particular subjects (called "topicals") like birds or aircraft. Some of the more popular collecting areas include: Postage stamps – particular countries and/or time periods Airmail stamps – stamps may be required for airmail, which