What are buyers and sellers (or customers and suppliers)

I’m programming a simple software to manage products, buyers/sellers and sales/purchases orders.

A product is simply a product, but what is the english word that refers to both the categories of buyers and sellers (or customers and suppliers)?

I just cannot find the correct term (if it exists), not even in my own language.

Some options I thought would be: ThirdParty, Person, Company. And finally, given the correct word, when I want to state if the ThirdParty is a buyer or a seller, what is it? A type?

I hope my question is clear and pertinent to english.stackexchange. Thank you.