I just got the Honest Hearts DLC a few days ago, and I have not played it much yet, but I am having trouble finding anybody to sell my loot to in Zion. So far I have encountered Graham and one other non-tribal (whose name I forget), but Graham's supply of caps was quickly exhausted, and the other man required that I have a higher speech or barter skill to persuade him to deal with me. Are these two the only merchants in Zion, or am I missing somebody? If there isn't anyone else, are there any safe places to stash my loot for the time being?

Yes those are the only two merchants. Store all of your gear in the reloading bench boxes in front of where you meet Graham in Angel Cave for the first time. Just keep collecting stuff, then once you have completed Honest Hearts, you can take what you're carrying at the time back to the Mojave with you, where you can store it at a safe house or sell it. You can then return and gather all that gear you collected, and return to Mojave again and sell what you don't want.

If you're low on caps, and need to buy something within Zion there is a boat load of Pre-War Money laying around all over Zion, use that and save your caps. Weapons and ammo, that are found in Zion are all sold at the Gun Runner's when you return to the Mojave, so there is no use in trying to stock pile up on guns and ammo for the .45 pistol and SMG.

You found them all... I played through Honest Hearts and had my stash in the bags where you first enter Zion and get ambushed... they were safe throughout my entire adventure, so that seems like a quick and easy place to store your excess items.

Lol, you beat me to the punch by 1 minute