If I want to buy/sell parts on FB Marketplace, how important is feedback, profile, and friends?

If you're trying to buy something, PM the seller and give him your phone number. If the seller doesn't want to talk to you on the phone, I'd quickly move on. If I can speak to someone on the phone, I can usually assure them that I am a legitimate buyer and will send them money (I use PayPal and cover the fees myself, NO FRIENDS AND FAMILY). If they won't accept PayPal I usually move on unless it's something that I really need.