Idea is simple. While youre in guild, you should benefit a bit more than what you benefit from it atm.
In guild guildvault. You put 2* cover in it, you can pull one 2* cover out of it. 3* lets you pull 3* that someone else has put there cause he doesnt need it.
Easy, simple and makes being in guild more important 
Also this would fasten on people getting those covers on those guys that they want to use
Trading in this fashion is too easily exploited. Create a dummy account. Join an alliance. Hack the account. Buy a ton of tokens. Dump them in the vault. Leave the alliance.
Loads of these types over in the suggestions forum. If the next Q&A thread ever pops up, i am absolutely sure a cover trading post or the like will be in it.
side note:
Moving to suggestions on 10/14
Ye i was thinking that there could be idiot rules like “you have to have roster amount of x” or “you have to be in guild for x amount of days”
Hacked account creates New Alliance. Floods vault, sits in New Alliance for x days.
Old Alliance moves to New Alliance, sits in New Alliance for x days, empties vault.