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.
Agent (see various dictionaries)
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.
I. A person or business.
- 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.