Heritage group The Letter Box Study Group (LBSG) estimates 200 are stolen each year.
Before 2003, Royal Mail sold or auctioned off postboxes that were decommissioned, which means some of those currently on the market are legitimate boxes that were sold off legally.
However, the large number available online on various websites has led the LBSG to fear the vast majority may in fact have been stolen.
A spokesman said: “There are legitimate boxes for sale but I would always assume that many have actually been stolen. Anyone looking to buy one should ensure they do not handle stolen goods.”
LBSG, which was created in 1976, acts as a database of postboxes across the UK and has identified around 800 different types.
The Royal Mail has introduced electronic tagging and permanent marking to identify stolen postboxes. It is also embarking on a renovation programme of all its 115,500 pillar, wall, and lamp boxes across the UK.
