This is actually a feature which appears in many other collecting-care type games, and is extremely useful.
The feature would be a global trade market where users could post individual items in request of credits.
You could post items for sale to the global market with commands like "t!market list brown_barrel 3000" to post a brown barrel to the global market for 3000 credits.
Also "t!market search item_name optional_sorting_choice"
^To bring up a list of those items users are selling.
Some optional sorting choices to the searches could be like--
low, high, average, new, old, etc.
This would help prevent many traps new members fall into by only seeing expensive things and not good prices. This would help open the trading community to more players, and would eventually help bring easier to see average prices to all items.