At land-based trade posts you can buy one of 5 freight carriers. Each one can carry a different amount of freight. The amount is presented as available weight. The help file Freight will give you an overview of commands and some information, but we will get into more detail through the trading section of trading.

An important note: When you have freight you cannot sneak, so most mobs will detect you in the wilderness if not invisible.

When in a formation the freight carrier will follow the form leader provided the owner is in the room. If you get separated from the form but are still in formation your freight will not follow your movements, you will need to unform or be made form leader in order to move with your freight.

A mule is a lightweight carrier, meaning it can carry very few trade goods. Heavier items such as teak might only take 1 unit but lighter items such as grapes, might take 7 units. Mules are handy for traversing most off road terrain types and can walk over shallow water and rivers. This is the only carrier that can traverse the perils of The Dark Woods. A mule is the most popular choice for trading through the dark woods, but remember that a mule CANNOT traverse swampy terrain.

Pack Horse

The lowest weight carrier available for use can also traverse a variety of off-road terrain, but it can also walk through marshy terrain. The UR-Ville trade post is surrounded by swamp, making this the only carrier that is able to reach this secluded outpost.

Open Wagon Horse

A medium-weight freight carrier which you will not see used often due to its weight limitations.

Covered Wagon Horse

Covered wagons are the most popular choice for small form or solo trade runs between land-based trade posts. It carries the most weight and subsequently the most cargo.  

Wagon train carrier.

The largest of the land-based freight carriers. As the name suggests, each wagon in a train will follow behind the leader's wagon in their own room.

TIP: Watch the movement points of yourself and your form. Low moves will result in losing players and breaking trains.

Buying Freight

Once you have decided which freight carrier you will use, the trade post operates much like a regular shop where you buy and sell items.
First buy your carrier e.g. Buy covered
Then buy your freight e.g Buy sugar.

To make this process less tedious, you determine exactly how many items to buy by over buying, e.g. buy 200 sugar, and you will get a message like this
The covered wagon horse will not hold all of that weight.
Only 34 more sacks of sugar will fit right now.
Buy 34 sugar will then buy the maximum amount of that trade good directly to your chosen carrier.

How to use mules and pack horses in conjunction with wagons.

As mules and pack horses carry very little freight, many players will trade larger quantities across the continent on wagons and then offload onto mules or pack horses to traverse the the open wilderness to places like Ur-Ville or Minataur post.
To do this, "hitch" your freight in the trade post where it will be safe and close to the trade master selling carriers. You can now buy a mule or pack horse and transfer freight

To transfer goods, "take <trade good>" repeat until you can transfer no more.
E.g. take emeralds

You can also specify exactly how many items to transfer.
E.g. take 11 emeralds

To get control of your primary carrier type, "take <carrier>"

Once you have reached your destination, simply "sell all". Don't forget to sell your carrier

You are also able to sell a specific amount of freight which is sometimes used to mix your goods from multiple trade posts. To do this type "sell x <trade good>" e.g. sell 10 sugar