Greg Manning began collecting stamps at the age of seven, and as a teenager he sold stamps through the mail. In 1966, at the age of 20, he held his first public auction. By 1971, he had opened an office. At the same time, he was operating a venture called International Postal Marketing, which was cited in 1981 by Inc. magazine as the fifth-fastest-growing private company in the United States. In 1993, Manning's ventures went public as Greg Manning Auctions, Inc. (GMAI).