So the MB is sweet but I got an idea while merching that I feel like would add a lot to the game and increase commerce in general which is good for the game's economy.
My proposal is this - allow us to post offers on items that are on the MB. How will this work? Well, say that somebody has posted a stack of 99 Toad Leather for 1.8k each or whatever. This would allow you to 'offer' on the item to tell the seller 'hey, I'll pay 1.2k each and you can sell them right now. So basically you'd select the item, hit Offer from the dropdown list and type in your offer price. This would notify the seller that they received an offer on one of their items. The would then go to their retained and select 'Offers' where you can either accept or decline the offer.
There would have to be limits on these like maybe you can post 5 offers at a time and receive 10 offer requests or something similar. This would *really* help people sell items more quickly especially on servers with lower population or selling items with high prices but low demand. Also, if you've blacklisted someone they can't post offers on your items (to prevent harassment) and you would be able to turn off offer notifications in the options and these wouldn't show up if your in a dungeon.
Also, you would have to have enough Gil to buy the items at your price in order to post an offer. If the person accepts the offer yet you no longer have enough Gil all of your posted offers will be cancelled and maybe you'll be blocked from posting offers for like an hour or so? I dunno, there would also probably have to be something like this in place to prevent possible trolling by making fake offers!