If you go to the buyers page, you can see "contact" under their logo. This way you can start a conversation with them. Hope it helps!
Usually, buyers contact me via messages on Export Portal. I get questions from potential buyers who are interested in my products (I am selling nuts) and we discuss all the order issues.
Being a buyer, I start the conversation to negotiate on prices, quantities or shipment options.
It is better to keep all the communication inside the Platform, as in case of any troubles you have the proof and may ask the EP Support to help you.