Online codes are included known as an online pass for multiplayer access or codes to download extra content. Beware of pre owned games that have had the codes redeemed already. Not every seller lists clearly whether it has or not. You should ask. To buy the access code often costs between £7 and £10. Some sellers will auction off unused codes separately, if you do make the mistake of buying a game with a used code - look for one on eBay separately.
If selling a game that has an unused code - never include a picture of the code as it could be redeemed by someone only looking at the pictures of what you are selling!!