Do not ship an item before you have guarantee that the money will come in. This has advantages, but one disadvantage is the lack of a great search function. For example, they may send an email that appears to be from Stubhub, complete with the logo and a long explanation of how you can wire the money safely to your seller with their special Stubhub protection program. Someone was offering up one of the sweetest looking kittens I had ever seen.
The seller requests you to immediately wire back the excess payment. Depending on your physical build and stature, you may be less prone to physical violence and damage, but it still sucks to walk into a transaction and have your item or money stolen from right under your nose. These scams happen all of the time. If you ever see a request for a wire transfer Western Union, MoneyGram, etc.