Go to paypal.com
Create an account linked to an email address
In your sale do the usual asking price etc, and when a sale is agreed move on to private messages (conversations) give your email address to the buyer and they pay into your PayPal
Note that in a sale the fees are deducted from what you receive, therefore Idealy decide how much cash you want, find out what uk postage would be - and check the insurance levels of different postage methods. Add those together and use an online fee calculator to see what you should ask as a total
Then you can advertise an 'all in price' making it much easier then saying £x plus postage and 'buyer pays fees' etc