While you don't need a paypal account to pay with pay pal you DO need one to recieve funds, they set up a direct debit to your bank account and then you can put funds in and have them transferred to your bank ( though they are quick to get money they are S - l- o - w to get money to you - 10 days). They offer the buyer some protection but can withdraw your money if there is a dispute and the vendor usually looses.... rare but possible.

My Pay pal account sends emails confirming reciept of funds, has a record of the sale and contact details of the purchaser and all the emails are from service @paypal.co.uk or paypal.com. I have never come accross an option to with hold funds until postage is confirmed and dont see how the buyer could have paid by pay pal if you dont have an account.....
sounds like a scam to me.

Where did you arrange the sale? Whatever way you sold do not post until you have funds..... the only way round the issue of trust is to use a website which confirms your trustworthiness in some way - ebay, ebid.
The best way to sell is locally, saving the cost of postage and cash can be paid once a buyer has seen the item working. Then sell sold as seen.
If its an old computer why dont you give it to someone using freeecycle - the sense of well being is much better than the stress of trying to sell 'an old computer' to a stranger over the internet - one of you has to take a leap of faith and risk being ripped off. Freecycle can become addictive and the genorosity of strangers restores your faith in human nature and is worth much more than a few quid.
http://uk.freecycle.org/