Follow these simple steps.

Step 1: Display an item at a normal price that people would buy.
Step 2: Stand next to that item while people are around it.
Step 3: When you see the (...), get ready.
Step 4: Once you see the simley face, open the menu, change the price to a much higher number.
Step 5: Go to your desk and sell the item.
Step 6: You will get the exact amount that you set it to.

Note: This glitch was performed on an unpatched version of the game. It is likely to eventually get patched.