Ramuh.Laffter said:
More like multiple items.
The idea is to find a server with higher-priced high-demand item, buy a bunch on your server at the lower price, then transfer to sell them on the other server.
Most do it with mules, but honestly, you end up waiting months to get the gil back to your server so it's not worth doing, in my opinion.
EDIT: FYI, the most common item used this way is Hagun. :P
An obvious solution
Create a mule on said server and transfer it back, then delete both.
But it's still in a way "paying" for gil, just legitimately. That 40 mil lets say you made, still cost you 40$