What is the best translation for the German word "Bündler", meaning an intermediary merchant for agricultural goods and produce?

Bündler does not appear in Duden, but from your description, a "Bündler" would depend on the exact nature of the buying and selling and would be qualified by a contextual specification.

Possibilities are

Agent (see various dictionaries)

OED

Middleman:

7.b. A trader, company, etc., that handles a commodity between its producer and its consumer.

1962 H. O. Beecheno Introd. Business Stud. xi. 93 What they have done is to bring all the operations, or most of them, under the umbrella of one firm and cut out various ‘middlemen’ as separate concerns.

wholesaler n. a person who sells goods by wholesale; a wholesale shop or business.

1888 E. Bellamy Looking Backward x. 146 The manufacturer sold to the wholesaler.

2007 J. E. Lloyd Hot Summer Nights iv. 88 Trish had surfed the Net and found a few wholesalers who could help her stock the store.

Factor:

I. A person or business.

• An agent for another.

1. Business and Finance.

a. An agent who buys and sells, or transacts other business, on behalf of another person or company, usually one based in a different place, on commission; a mercantile agent; a commission agent.butter, cheese, cloth, horse, malt, timber, wool factor, etc.: see the first element.

1850 W. Irving Mahomet & his Successors I. v. 49 Mahomet..was employed by different persons as..factor in caravan journeys to Syria.

1954 G. Ohlin tr. E. F. Heckscher Econ. Hist. Sweden vi. 246 The merchants could now hand over the task of transportation.., while previously they or their factors had staked lives and fortunes on the delivery of goods.

2010 D. L. Brady Essent. Internat. Marketing xi. 193 A foreign factor is a type of agent that sells consigned or delivered merchandise for a commission known as factorage.

The term food distributor is relevant.

From Treehugger (bolding added):

Plenty of small farmers get by just fine without ever selling produce or other small farm products to a food distributor. But some farms find it's a great way to market their products and keep income coming in steadily on the farm.

Definition:

Food distributors act as the middleman between the farmer and the customer, or retailer. They purchase farm-grown products directly from the farm, then sell them to a variety of customers: restaurants, grocery stores, and supermarkets, schools, institutions like hospitals and universities, food processors, and food manufacturers.

And from LearningHub:

What is food distribution?

In general, food distribution is composed of a variety of companies, organizations and programs that collect food from producers, store it in warehouses, and then distribute the food to manufacturers, grocery stores, restaurants, cafeterias, government aid programs and more....

How does food distribution work?

In general, food distributors handle transporting and storing food as it travels from producers to food service operators. Some distributors specialize in certain food products such as seafood or fruits and vegetables. Some may work for a specific food manufacturer. Most food distributors are independent, which makes it easy for manufacturers and producers to create a supply chain by simply hiring a distributor that caters to their specific needs.

The inclusion of seafood means that 'food distributor' is a hypernym, although usually most food originates with farmers / growers.