Instead of taking delivery of a grain silo's worth of wheat (really, a tremendous amount of wheat), the farmer agrees to let you store the wheat in their silo until you have a final buyer. Then, using a bill of sale, you sell the final buyer the right to the wheat.
The final buyer, with the bill of sale in hand outlining what they bought from you, can then go to where the wheat is stored, present their bill of sale, and collect the wheat from the farmer's silo. In some cases, you won't be the trader to find the final buyer.
Instead, you may sell your bill of sale (your right to an amazing amount of wheat) to another trader, who may sell the bill of sale again to yet another trader until it finally finds its way into the hands of the owner of a shredded wheat factory. This simplifies commodity trading since you only have to sell the rights to the property and don't have to move the property multiple times.