It is a money laundering scam.
Money laundering is converting worthless money into real money.
Scammers target Craig's List, Auto - Power Sports - and Boat Trader, and other online "sell your stuff" site users in an attempt to get you to send them money.
Here is how it works:
They say they will buy your item, unseen, and have it shipped to their location.
They will pay you with a cashiers check, money order, or certified check. Others also say they will pay via PayPal or any other online cash-transfer service and ask for the email address you use for that account.
They say they will pay extra to cover the fees to ship the item to them via the shipper/agent they use.
They instruct you to send the extra money they send to the shipper/agent via wire transfer or prepaid money card.
You receive the check for the amount over your item and cash/deposit it. For the PayPal scam, you receive an email invoice from PayPal showing that the funds are deposited into your account.
So, you use your money to send off the funds via wire transfer or prepaid card to the shipper/agent as instructed.
.....then, no one comes to pick up the item.....
A few weeks later, the check bounces, as it was a counterfeit, forgery, or stolen and it takes many days for the banking system to process and find this fact out. For the PayPal/Online Money Transfer, you find out that the email invoice you received was a fake, there were no funds deposited into your account.
As the money you had sent was via wire transfer or pre-paid card, it is unrecoverable and untraceable.
You are out the money you sent, the bounced check amount, fees as a result of the bounced check, and can be held criminally liable for passing a counterfeit/forged/stolen check. For the PayPal/Online, you are just out the money your sent but the scammer can also attempt to hack your account.

The scammer has used you to launder the fake check or fake PayPal into real money, at your expense.

If any of the people reading this have found themselves with the fake check in hand, DO NOT CASH OR DEPOSIT IT.
DO NOT take it to a bank to get inspected or to have the accounts checked to see if they are valid.
DO turn over all checks and correspondence to the US Postal Inspector or US Secret Service office in your area. Have a police report filed for your protection.