Word describing people who need to establish trust, But aren't friends

"Counterparties" is the word that is used in the financial industry for parties at either end of a transaction. It's most frequently seen as part of the phrase "counterparty risk", which is the risk that a counterparty won't be able to hold up its end of the bargain.

You can consider interdependent.

relating to two or more people or things dependent on each other

^{http://www.thefreedictionary.com/interdependent}

There is also a concept called buyer-seller interdependence in business markets.

the close relationship between buyers and sellers, especially in organisational markets, where buyers become highly dependent on sellers for assured supply and delivery.

Monash Marketing Dictionary

Furthermore, Wikipedia lists types of dependencies and explains the uncertainty and level of trust in social network interdependencies as below:

Uncertainty: increases when perceived dependability and reliability of informal ties are lower. This is related to the level of trust that exists between the parties. Other authors like Sheppard and Sherman^[26] believe uncertainty also differs depending on the form and depth of the relationship. Consequently, they proposed four levels ranging from ‘shallow interdependency’ to ‘deep interdependency’^[26]

^{_{26. Sheppard, B. H.; Sherman, D. M. (1 July 1998). "THE GRAMMARS OF TRUST: A MODEL AND GENERAL IMPLICATIONS."}}

Based on the narrow case of a business relationship that consists of just one transaction, I don't think there is a single word answer. In the single transaction, buyer and seller would rely on verification, not trust. In other words, the buyer would inspect the product carefully (or have a trusted independent inspector evaluate it), and the seller would either demand cash or have a trusted independent authority (bank, etc.) verify that the buyer's means of payment (credit card, check, etc.) was legitimate.

However, in the case of a recurring business relationship, verifying every transaction might be inconvenient. For example, if I sell you a truck load of apples every week, it costs a lot of time for you to inspect each apple, and costs me time to make sure your check is good. In this case of recurring business, @ermanen is correct: "interdependent" is the right word.