Stop telling all my secrets Albus...
As Albus mentioned Moonito, you have to have 1 person playing for each pc with an open client.
As for autotrade, Albus did well again. The merchant has to be on another account as the command @autotrade or @at pretty much gives the job to the server to maintain the shop. If you log on the account in question, it will disconnect your shop and you have to log once more to get on a char.
If you have more questions, don't hesitate to ask.