Earlier quoted context omitted.
Law enforcement could have easily MITM'd the PGP. They replace the public key of a vendor with their own public key (on the vendors's page, without the vendor's/buyer's knowledge), then the buyer address gets encrypted with that public key. Then they decrypt and resend the message using the sellers original public key. I really wonder if this happened at all
They almost certainly did. I do market research and a number of vendor keys changed on Hansa and Dream on June 23rd. Some had various comments in them claiming to be older, e.g. "EST June 2016". Dutch Police released a statement claiming as much http://politiepcvh42eav.onion/hansafaq.html
Maybe this was done on a case-by-case basis or Grams updated their keys to the new LE keys.