Buying from customer?

I’m in a business where I buy and sell bulk material from businesses and the general public. The people that I sell to are a small number and generally don’t change. What I’m interested in is how I can go about tracking relationships with the people/businesses that I buy from. I have a list of what I’ll call leads, because they’re people that I want to reach out to, build a relationship, and do business with. But I’m buying from these people. I can’t convert leads to suppliers.

Alternatively, I could keep them as customers but add a customer type. But then I’d want to be able to record purchases against customers, and not just suppliers.

It would be nice if there was a more general “account” doctype, where we could enable/disable purchasing and selling.