My guess from context clues:
Prospective buyer: “One drugs pls to this shipping address.” Seller: “0,05 ETH please at this wallet address” Prospective buyer: “Test amount sent, pls verify” Seller: “I see your test amount. Shipment will be sent once full amount clears” Prospective buyer: “Thx for verifying. Remainder on its way.” Seller: “I see it, thank you for your business. Shipment of one drugs will arrive in two to three weeks. Enjoy!”
I imagine these sorts of messages between buyers and sellers which are hosted on the platform would be the PGP encrypted messages in question.
The USA and UK are both Five Eyes members and routinely share intelligence. If the USA had compromised secure apps, wouldn’t the UK be privy to intelligence gathered via that method?