We have the bounty hunter theme, Sentinels, Hellknights, spaces pirates, space slavers, and other assorted ways for people to get into mischief and people who want them caught.
How would you go about assigning/claiming bounties? Ideally, it's based on the crime vs. a metagame consideration like WBL but that could provide some basis as a higher-level offender can probably pull off more villainous/higher-profile offenses.
Coincidentally, this is as much a question for applying bounties to PC law-breakers as it is for PCs to collect on bounties.
How you handle this will depend on if it's the PCs intention to focus solely on being bounty hunters, and how quickly you intended for PCs to level up.
I can't recall the base structure. My group stopped using XP a long time ago, and we also implemented a system where you automatically have WBL at each level up. You can gain items for use in between levels up by picking them up off an enemy or finding it etc, but when you reach level up you have to decide if you want to keep and have it cost against your total WBL or chuck it. It means that we don't need to keep track of loot unless someone specifically wants the item, which is nice.
Honestly, the easiest approach is that any CR rewarsd the party would get from an encounter of that level comes most from the bounty, minus any value from gear that the PCs may claim off of the enemy.
In my campaign there is a bounty posting service called Double Blind based in the spike on Absalom station. People can post bounties on individuals, and through very secure encryption, neither the poster or the bounty collector can find info about each other. However there is a rumor that there is a Ysoki hacker in the Down Low that has discovered a way to crack the code.
That seems problematic.
If anyone can easily see that someone has bounties on them, then the Absalom Station security would probably be looking out for those people, and also trying to shut down the business/net location (assuming it's on the net). And if it's not on the net, then people would be involved to communicate the information and it wouldn't be very secure.
Also, bounties usually aren't illegal as your suggesting. Bounties were usually provided by legal organizations, like police, for apprehending someone they couldn't due to whatever myriad circumstances. Strictly speaking a bounty doesn't need to come from a legal authority, but openly posting bounties that flout the law would cause lots of problems.
What you're describing sounds less like a bounty service, and more like a Hitman service.