This prank is absolutely hillarious and probably the only scam in this game that I approve of.
Step 1: Mail a letter from one character to another character (Mr. Smith to Mrs. Smith in this example). Be sure to write something funny on the letter, this is where your prank goes.
Step 2: Go log off Mr. Smith and log into Mrs. Smith to collect the letter.
Step 3: Take the letter out of your mailbox so that it's in your bag space.
Step 4: Post the letter on the auction house, be sure to check your auctioneer to see what a good price would be for it. Post it for around 7% or less of the nearest price.
If you don't have auctioneer, just check the current rate of Plain Letter (s) on the auction house. As long as you're way below the average auction house price your fellow auctioneers have a good chance of mistaking the item for an incredible deal.
Please don't abuse this, but it is very funny and I suggest trying it once and keeping it below 5 gold. I did this to two people on my server just to try it out and refunded them the five gold each, it was pretty darn funny.
In order to set up the prank, you may need to post several letters at incredibly inflated prices for a few days.
Be sure to leave what you would write on the [Plain Letter] in the comments section :)