It may be a scam.

Scammers use Craigslist and other online buy/sell sites to troll for victims.
Sometimes they request to do the transaction through PayPal or other online money transfer services and some use fake or stolen checks.
Bottom line, they have no interest in getting the item you are selling, only in tricking you into sending them money.

Look for the signs of a scam in the examples below:

Here is how the "PayPal Scam", or any other online money service money-laundering scam, works:
They say they will buy your item, unseen.
They say they will pay you extra money to cover the shipping charges through their shipping company or agent or to cover any fees.
They request your email address linked to your PayPal account.
They instruct you to wire the extra funds they send you to their shipping company or agent or person who collects the fees.
You receive a invoice from PayPal to your email, stating the funds have been deposited to your PayPal account.
So, you use your money to wire the funds to the shipper.
You then find out the PayPal email was fake, it was sent by the scammer, there were no funds deposited into your PayPal account.
The money you wired, was really picked up by the scammer, it is unrecoverable.
You are out the money you wired.

Here is how the Cashiers Check, Money Order, or Bank Daft scam works:
As above, they want to pay you extra to cover shipping or fees.
In these cases, the check you receive, for funds over the amount of your item, is forged, stolen, or counterfeit.
As above, you are instructed to wire the extra funds to another party.
As it may take several weeks for the banks to realize the check is bad, you may not know you have been scammed for some time.
Worst yet, as you passed a bad check, you can be held criminally liable.
You are out the funds you wired, return check charges, and face possible arrest.
See:
http://www.fraudaid.com/
http://www.fraudaid.com/solution_center/solution/draftsoption3/4c.htm
Turn over any checks and letters you receive to your local Postmaster, Secret Service field office, or local law enforcement.
Do not try to deposit or cash them.