Used stamps exchanges.

CTO's:

A cancelled to order (also called CTOs) postage stamp is a stamp the issuing postal service has cancelled (marked as used), but has not traveled through the post, but instead get handed back to a stamp collector or dealer. They can come from withdrawn stocks of stamps cancelled in sheets and sold as remainders or from new sheets for sale at reduced rates to the stamp trade. Postal services of various countries do this in response to collector demand, or to preclude stamps issued for the collector market being used on mail. Some of the history of CTOs is from stamps being given to collectors on an approval basis, in person or through mailings; the first CTOs began in the late 19th century.

here is a list of 105.000 definitives
http://colnect.com/en/stamps/countries/emission/1-Definitive
it might be interesting for you to look at it

Commemorative stamps:

A commemorative stamp is a postage stamp, often issued on a significant date such as an anniversary, to honor or commemorate a place, event, person, or object. The subject of the commemorative stamp is usually spelled out in print, unlike definitive stamps which normally depict the subject along with the denomination and country name only. Many postal services issue several commemorative stamps each year, sometimes holding first day of issue ceremonies at locations connected with the subjects. Commemorative stamps can be used alongside ordinary stamps. Unlike definitive stamps that are often reprinted and sold over a prolonged period of time for general usage, commemorative stamps are usually printed in limited quantities and sold for a much shorter period of time, usually until supplies run out.[1][2

Definitives:

The most common stamp of all is called a definitive stamp.
In it's simplest definition, a definitive stamp is a stamp that is issued to serve as a handy, all-purpose stamp, and is produced for an extended period of time (weeks, months or in many cases, years). They are almost always part of a series of stamps, called a definitive series, where a wide range of denominations are issued. The entire series may be issued at one time, or they may be issued over a span of years.

Most definitive series of stamps concentrate on a specific person or office (such as a president, monarch, or prime minister) or a specific theme (such as flags, buildings, or occupations).

Most definitive series will be denominated in such a way that any combination of postal rates can be met. Think of them as the work-horse of the postal administrations.