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.