It is a scam.
They will want to "pay" for your item via PayPal or other online money transfer services, because they know how to trick people into thinking they got paid. They don't want your items, just your money.

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.
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 to use pre-paid money cards like Green-Dot or such.
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.

The same scam is also done with money orders, bank drafts, or cashiers checks.
In these cases, the check you receive, for funds over the amount of your item, is forged, stolen, or counterfeit.
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

It is also done with stolen credit card info, in these cases they generally press you harder to act faster, as the time it takes to find out the cards are stolen is much smaller.
However, the scam remains the same; They get you to send money via wire transfer or pre-paid money card before you find out the card info they provided is stolen.
Result, you are out the money you sent them.