Create a second character known as a "mule". With this new character sell all his/her(or its) equip on the Auction House and also trade the Adventurers coupon (given to you by the NPC at the very beginning) to the appropriate NPC.
Then simply send the money to your character. This is a very cheap way to earn gil, but not a lot of it (the most I could get was about 1,000).