Bonds are in-game items which can be bought from Runescape/Jagex. Jagex launched bonds in order to fight massively growing real world trading. The players can buy bonds for like $6 and then sell it for RSGP in the Grand exchange or to another player directly. They can only be traded once after they have been bought, once they have been traded, the bonds become untradeble. In order to make them traedeable again, a small fee of around 1.2M~ RS3 gold is to be paid. Bonds are traded in mass amounts on daily basis throughout the RS3 and Oldschool. At first, only Rs3 bonds were released but after their success, Jagex made bonds in oldschool too. Unfortunately, the oldschool bonds can only be used for 14 day membership rather than the RS3 bonds which can be used for membership and many more perks in-game.
Add my Skype: Matt_740