FWIW, your first example would be rendered moot if people used PGP.
it would not resolve the issue. Someone could provide the PGP signed message in lieu of chat log screenshots. When someone gives a PGP signed message asking to change the payment address the first one would still not have a tx showing payment.
You would have the same issue of having a locked thread without the ability to give a rebuttal.
Maybe you should write out your example in full, because you just lost me. If I have two pgp-signed messages from a seller saying "send money here" and "send money there", what's the problem? Send the money to the newest address they gave you. Create a scam thread with the PGP messages if they scam you.
1- buyer and seller agree to a trade
2- sends a widget to the seller
3- seller gives PGP signed message saying to send funds to address (a)
4- buyer receives widget
5- seller gives a 2nd PGP signed message saying to send funds to address (b)
6- buyer sends funds to address (b)
7- seller opens scam accusacation saying they were never paid
8- seller provides the PGP message for address (a) as well as screen shots showing he gave that address to buyer
9- seller locks thread before buyer can respond
