We tested it awhile
back, I was able to edit the messages after you'd already read them in a conversation. Personal opinion, I'd just treat conversations like SMS or email and make them lock once sent, people can just follow up with correction messages if they mistyped something or decided to change their mind.
There is no formal commitment for agreements or purchases that I've seen here anyway, and none are posted in the rules in the trade section, so if anything people just learn who are flaky sellers and avoid transactions with them. Different story if you already sent payment but there seems to be a process for that.