DarkEnergy >> Every trade has a buyer and a seller
Correct - stock exchanges treat each buy/sell as a 'single transaction' and its up to software systems like 'ADVFN' and 'London South East' to individually calculate whether it's a buy/sell depending on the bid/offer price at the time
This method is a best guess estimate which is why some systems will show it as a sell whereas others will show it as a buy