We apologize for the inconvenience, but this should have been enforced all along. The point of this is to make the post un-editable. The goal is to prevent someone trying to buy/sell an item and the OP changing the price after agreement but before delivery.
This has been written into our market rules from day 1. We apologize for the inconvenience; however, we have to do our best to balance all parties' interests in the marketplace and mitigate scam risk. We believe this is the best balance.
Short answer: Nothing that allows you to bypass the "no-edit-post" rule in the marketplace is allowed.
Seems we've had some mix-ups in our internal communications. We'll make sure all moderators are clear on this for consistent treatment.