A number of RTS games have the option to send your teammates resources(there's usually a penalty associated with this).  Obviously it can be done with buy an item, drop it, teammate picks it up and sells it for 80% of the cost.  However, this is a team game so it should include mechanics that reward working as a team(and sharing resources is obviously one of these mechanics). 

An easy way to implement a gold sharing mechanic is to have a team piggy bank at the citadel that allows anyone to deposit or withdraw a specified amount of gold.  This would allow for greater cooperation in buying both citadel upgrades and items.  For example, a team could pool their gold for an early heart of life, and then share it with each other so they all get the effect.  It would just lead to a greater potential for cooperative teamplay to have this kind of feature.