Complex sentence: Standing at football (soccer) matches across the globe is very common, and tickets sold as standing area tickets are sometimes the most popular, i.e. they are not sold merely when all seating tickets have been sold out.
Split into simpler sentences: Standing at Football (Soccer) matches across the globe is very common, and tickets sold as standing area tickets are often the first to sell out, and amongst the most popular. They are not merely sold when all seating tickets have been sold out.