I had an idea similar to this once, but didn't want to post it for fear of flames.
Here's my idea;
Credits overflow into the armoury, if a player wishes to buy an item, but is short a few credits (he/she has to have atleast 3/4 of the credits neeeded to buy the item) the bank will give the player the necessary credits. Selling back the item for full value wouldn't be too profitable, the most you could get is 150c more from selling a luci (if you bought it with 3/4 of the credits, no more).