Money payed in advance, is it a scam?

So I'm posting this as I'm worried about my friend getting scammed. He has put a computer up for sale on gumtree for £900 and was contacted by someone who wanted to buy it. They wanted to send the money by PayPal and send a courier to collect later in the week. My friend has accepted the payment and the guy sent him £1000 (extra £100 to take the advert down) and he actually has the money in his account (can spend it and everything). So is this a scam or not. It seems to be very fishy to me but I don't get how this scam would work, with the money already in the account and spendable etc.

Did he/she actually sign into Paypal not using any links and verified their is money in his account? Did he/she call Paypal and verify it? Or, is he/she just reading an email.

It reeks of a scam. Who pays more to remove an ad? Who pay's a courier to pick up a computer.

A real person would be better off buying a new computer at a retail outlet.

Has he been told to send money to the courier? If so, it's an overpayment scam, in which the scammer has either sent a fake PayPal payment or gotten control of someone else's PayPal account, and the courier is a character played by the scammer.

Did he/she actually sign into Paypal not using any links and verified their is money in his account? Did he/she call Paypal and verify it? Or, is he/she just reading an email.

It reeks of a scam. Who pays more to remove an ad? Who pay's a courier to pick up a computer.

A real person would be better off buying a new computer at a retail outlet.

He did sign in to PayPal the normal way, not using any links and the money is actually there. I also thought the same about the removing the ad, who has a spare £100 to chuck round right.

Has he been told to send money to the courier? If so, it's an overpayment scam, in which the scammer has either sent a fake PayPal payment or gotten control of someone else's PayPal account, and the courier is a character played by the scammer.

No he has not been told to give money to the courier however the money is actually in his account. I hate the thought of some poor guy losing a grand like your saying (if the scammer has got someone else's PayPal).

Can you post the emails involved, including full headers? Omit anything that would identify you or your friend, of course.

Can you post the emails involved, including full headers? Omit anything that would identify you or your friend, of course.

I can't get hold of any emails unfortunately as my friend seems to think it's a good idea, that's why I've posted here to find out what type of scam it is then I can say to him this is what's gunna happen to you if you do it.