Who is the buyer and who is the seller?

Whoever creates the deal chooses. The buyer (or payer) is the one paying money into escrow. The seller (or payee) is the one delivering and getting paid.

It is shown on the deal at every step so nobody is confused. Only the buyer can fund a deal, and only the buyer can release the money. A seller cannot fund their own deal and cannot pay themselves out of escrow — that is the whole point.