Contacts Who Are Buyers AND Sellers. How to track deals?

Contacts Who Are Buyers AND Sellers. How to track deals?

Hello,

I have a list of contacts who are mostly buyers. On rare occasions, these buyers also act as sellers. I currently have all contacts saved in the Contacts module and I have a custom field named "Contact type", where the selections are either Buyer or Seller. We would like to track all deals related to the contact, but we need to know when the Contact is acting as the Buyer and when the contact is acting as the Seller. What is the best way to execute this? 

Thank you in advance,
Anthony