In Chapter 1056 we discover that Buggy has joined forces with Dracule Mihawk and Crocodile in a group called Cross Guild. The Cross Guild is capable of issuing bounties on Marines.
I remember this theory according to which there’s a secret organization with bracelets which is an x or cross. This image is old news, everyone who has followed One Piece for some time has seen it. We see that Doflamingo, Mihawk and Blueno wear a peculiar bracelet of some sort.
So in theory since Mihawk got one and also Doflamingo as seen in picture there might have been a mutual agreement between Shichibukai if ever betrayed. Perhaps Doflamingo could be freed from Cross Guild. Blueno is the only one even back then I don’t know why he wears it, however this image may have foreshadowed Mihawk joining the Cross Guild.