How to structure a "many-to-many" relationship between nodes I would like to have 2 node types, Sellers and Products. Each seller can have many products, and each product can have many sellers. Also, the relationship between Sellers and Products would need to contain a price.

What would be the best way to create that relationship, making it easy to list all the Products a seller sells on their page (with prices), as well as listing all of the Sellers selling a product on their page (with prices)?
You can achieve by these two modules:
Field Collection and Entity Reference,

You can create Field Collection field (unlimited instances) to Product, the collection field (act like a wrapper) contains an Entity Reference field references to Seller, and a price field for that seller. 

Some screenshot for you: