Go to the shipping screen.
Choose an item (Press ''A'') that you have A LOT of.
Move to the maximum # that you have for that item.
Press ''B'' to cancel away.
Choose an item (Press ''A'') you want a lot of, whether it be expensive of something you need for a job.
Press Down once. This number should be many more than what you actually have, usually appears as one less than the total of the item you wanted to sell.
Press ''A'' and sell that amount.
Go back into the screen, and notice you have many more than you did. Repeat if desired.

NOTE: This trick is many times more effective if there is a larger amount between items (55 and 1)