The Easiest Way to Trade in Person
If you want to buy or sell an item fast and do not see it on the Market then you can advertise in whatever city you are in or on the Advertising Channel. If you get a buyer/seller who is in another city you do not need to take a Boat or Magic Carpet but can simply walk to the Temple in your city and from there travel to the Adventurer's Guild.
The Adventurer's Guild is on an island that is a Protection Zone and it can be accessed by using an Adventurer's Stone inside any Temple. If you do not already have a Stone then just ask the Temple NPC for an "Adventurer's Stone" to get one for free. There is a Banker NPC, Kepar, so you do not need to carry money to the temple and a Postbox so you can mail your items to your Inbox but make sure to bring a Parcel and Label.
Remember to be careful of PKs between the Depot and Temple.