In regards to your 6/22 post Trib saints can buy and sell without the markergrou !!! in which you stated 
underground people should be able to buy and sell without the mark during the tribulation period, are you referring to people living underground, such as in caves, who are buying and selling, or people who are buying from a black market that is known as an underground black market although it may not be under the ground? Or maybe both?  Well, if you are a trib saint living in an underground cave, you would have to come out of it sometime in order to barter with someone who is your supplier. However, don't forget that there probably will be thermal drones to track you once you are outside your hiding place of a cave, or if the person bartering with you comes to your cave, the thermal drones will be able to track them down with heat sensors because they can be used for hunting with the trib saints as their game.  And if you build a fire outside, the thermal drones will be able to see that as well.