There are several factors involved in trading.  First is acquiring the goods, second is transportation, third is selling the goods at your destination.

Buying/Selling Goods

When buying or goods, you attempt to bargain to get a good deal.  The bargaining is an opposed Persuasion roll between the buyer and the seller.

Each point of difference between the rolls results in a 1% difference in the base price of the good.  Various factors also effect the value of the goods when you sell them at a location.  Distance from their source, rarity, demand for the good in the location and other factors.

Transporting Goods

Goods are usually transported and sold in 50 lb units.  When transporting goods, there are 2 factors, bulk and weight.  All trade goods have a bulk/weight rating.  A 1/1 rating means that 50 lbs of the good take up 50 lbs of space.

When you are calculating how much goods you can move, you have to use the weight OR bulk value – whatever is higher.  So if a good is 2/1, then each 50 lb unit takes up 100 lb of bulk.

How Much can you Move?

Animals have a weight/bulk amount that they can move.  If the animals are pulling a carriage, cart or wagon, they can move 5x their weight/bulk amount,  including the weight of the vehicle.

If there are multiple animals attached to the vehicle, add their weight/bulk amounts together.

So 2 mules (400 lb capacity each) pulling a wagon (which weights 400 lbs) could move (800*5-400) 3600 lbs of goods.  Or 72 bulk/weight units.

REMEMBER – you need Vehicle tool proficiency to transport goods by vehicle, and you have to make Animal Handling skill checks when transporting goods on animal-back.