1P can be translated as venda direta. It literally means "direct sale": a company selling directly to its consumers.
3P can be "translated" as Marketplace. I know, it's not in Portuguese, but it's the word being used by B2W (and other players) to describe a system where a third-party sells the product using another party's infrastructure:
O B2W Marketplace é uma plataforma que permite que você venda seus produtos nas vitrines da Americanas.com, Submarino e Shoptime. A B2W funciona como um mediador entre você e o cliente, oferecendo o tráfego mais qualificado da internet brasileira e a melhor experiência de compra.
Free translation: B2W Marketplace is a platform that allows you to sell your products at Americanas.com, Submarino and Shoptime. B2W works as a mediator between you and the client, offering the most qualified traffic of brazilian internet and the best shopping experience.
Notes:
- In the corporate world (at least in Brazil) it's very common to use English words to describe something instead of trying to find a proper translation. And it seems to be the case with Marketplace - I believe that's because Amazon also uses this word and nobody could find a good translation to Portuguese.
- I've also seen Programa de afiliados being used, by I think it works in a different way if compared to a Marketplace. And I think that Marketplace fits better to Amazon's definition.
- I think it also depends on your audience (if the term is familiar to them). If it's not the case, you can use venda por terceiros (third party sales). Although I personally don't like it, I believe you'll be understood.
