Drop trading is the term given to the trading method by which one player drops an item to give to another player, and, after waiting for the item to become visible, the receiving player picks it up.
Historical drop trading
Drop trading has been used historically as a way of breaking Rule 6, which dictates that no items may be transferred between players of the same owner, usually a Main account and a pure. This use of drop trading was used when having two clients opened at the same time was not possible.
Drop trading was used even more before banks that could hold items were introduced to the game. Many players would have "mule" characters which would serve as an item bank, and transfers would be done through drop trading. This was highly against the rules and many players have been permanently banned for it. Though this is hard to catch due to the player not saying anything during the drop trade transfer. However, drop trading is also risky because there is a chance that the items could be intercepted by another player.
In the very early days, this rule was rarely enforced. Well known players often asked for "coal mules" in return for expensive items. It is reported that Jagex started deleting mule accounts some time in early 2001 but stopped due to the chance of an innocent player being deleted.
While the Trade limit was in place, only items of small value could be seen by other players.