See Bounty: Gemstones for an explanation on how Black Market "Near" boosting works.
These particular bounties are Spanish gold, which is basically gold coins. They are found between chapters 21 and 27. I farmed these bounties during chapter 21 with Sully's gun. You will eventually reach a part where you have to shoot enemies with Sully's gun in the boat. Simply kill them all until you reach the final guy on the left before the rope bridge just in front of you. Once you've done that either restart the checkpoint or back out to the main menu and press continue. I found the second method worked best. You can see how many you have by pressing 'Black Market' on the main menu. The trophy will unlock after getting all the bounties in this set.