IT would be really nice to have buy and sell signs for players to use. It would work by players putting a sign on a chest with an item and creating a sign like this [buy/sell]
item name
amount
price
You would only be able to buy and sell player to player as no not break the market. Someone could make a shop that sells iron blocks at they base for people to buy. Buy sign would mean when you click the sign you would buy {duh} and sell well sells. Also It could work with ranks so a normal player could say have 2 shop signs while a with ranks you would get more or you could add more to the buycraft shop.
An example of this plugin would be https://www.spigotmc.org/resources/signshop.10997/ https://dev.bukkit.org/projects/signshop