Skinflint is the trader that can be found in the main building of the Freedom faction headquarters. The first time you talk to him he'll give a mission that requires you to eliminate the Bloodsuckers in the nearby village (unless you already picked up the same mission from Murk. Skinflint will be selling items dependant on your "status" with Freedom - if you have done missions for them he's very likely to sell better and more expensive equipment (such as the SVUmk2 sniper rifle and the Exoskeleton armor).
It seems that after the 1.0003 patch Skinflint will now also sell you 7.62x54 mm ammunition for the SVDm2 and the SVUmk2 sniper rifles.