No doubt they will want to pay the shipper by cashing that check and sending the money by untraceable means such as Western Union. The check bounces and you are out the money and the bank fees.
This is the way these scams works:
They say they will buy your items by check, money order, or cashiers check.
They say they will pay extra to have the items sent via the shipping agent they use.
They send you a check for way more than the amount of your items and instruct you to wire transfer or pre-paid money card the fund over the purchase price to the shipping agent.
So, you deposit the check and use money you already had in your account to wire/pre-paid transfer funds to the agent.
Oddly enough, no shipping agent or company comes to pick up your items.
Weeks later, the check you deposited bounces as a forged, counterfeit, or stolen check and the face value of the check plus fees are removed from your account.
You then find out the money you wired/pre-paid to the "shipping agent" was really retrieved by the scammer. As the fund were wire/pre-paid, they are untraceable and nonrecoverable.
You are out the money you sent to the scammer, fees for the returned check, plus depending on the financial institution you use, you may be criminally liable for depositing a fake/stolen check.
Money laundering = Making worthless "money", as in the fake/stolen check in this case, into "real" money, as in the money you sent to the scammer.
If you receive a check from the scammer in the above circumstances, turn over everything to the Postal Inspector or Secret Service Field office in your area. If you have already deposited or cashed the check, file a police report and get with the financial institution you used to work out a deal to get the accounts balanced out.