Then users looking at their accounts from other logins would see the wrong public key and know something was wrong. Also wonder if that happened at all
The only reason I mention that this is at all probable is because of the length of a PGP key. How often is the average user of a site like this logging in to verify even the last few bytes of a PGP pub key compared to what is saved in their software? Plus how many users would chalk it up to "oh SellerX just changed their key pair" and continue on encrypting their message with the new key
I highly doubt anyone looked at it or cared. People were most likely on there to buy drugs. Do you think they turned to their friend and asked "hey do you mind looking into this website and comparing the PGP keys with me? I just want to be sure!"