These programs (and other similar variants) allow a large number of items to be kept in stashes away from your D2 character. You can use ATMA or GoMule, both are a ways of moving items around (they include other feature such as a drop calculatir and unlimited stash capabilities). The easiest way to transfer items between character in single player, is to use a third part If you change characters more than 4 times in less then 5 minutes you will receive a temporary ban ('Realm Down'), by the time you get back in after a temp ban your game will most likely have expired.

Once the game have been active (at least one player in the game) for over 5 minutes, it will stay up for 5 minutes even if there are no players in the game.

The safest way to solo transfer items between characters ('muling') within one account is by creating a game (I recommended you create a game with a password, so no strangers or spam bots wander in) and waiting 5 minutes before dropping the item in game and logging in with another character to pick it up.