What does the green creep want?
Solution 1:
these creep just wants items as weapons or passive items. if you drop one item just were he is,he will give you coins in return. if you really need coins, break a chest, it can drop trash. giving the trash to the creep, you will get 3 coins.
Solution 2:
The Sell Creep has a chance to appear in any normal shop. If items or guns are dropped on the grate, the Sell Creep will take them and give Coins in return.
http://enterthegungeon.gamepedia.com/Shop#Sell_Creep