One word that describes customer, business and vendor [duplicate]

Solution 1:

Party (MW dictionary):

3:  a person or group participating in an action or affair 

a mountain-climbing party

a party to the transaction

Examples of party in a sentence:

the two parties in the marriage contract

The parties in the lawsuit reached a settlement.

Solution 2:

stakeholders

Quoting BBC for Schools site:

A stakeholder is anyone with an interest in a business. Stakeholders are individuals, groups or organisation that are affected by the activity of the business. They include:

• Owners who are interested in how much profit the business makes.
• Managers who are concerned about their salary.
• Workers who want to earn high wages and keep their jobs.
• Customers who want the business to produce quality products at reasonable prices.
• Suppliers who want the business to continue to buy their products.
• Lenders who want to be repaid on time and in full.
• The community which has a stake in the business as employers of local people. [...].

Solution 3:

You could also try 'actor'. It is formal but according to Merriam-Webster Collegiate Dictionary, 'actor' means '[...] 3: one that takes part in any affair'.

Or you could also try 'transactor' – from 'transaction', which would include purchase/sale.