Buying and selling stuff on the battlefield is not going to go well.
Because few folk bring cash onto the battlefield, and while there are a LOT of valuables taken through the Sentinel gate, these tend to be for use. Magic weapons, potions, mana crystals for battlefield rituals and the like.
You COULD turn up with a huge stash of potions, stand behind the lines, and try to sell them. At which case you’d be accused of various things. It’s assumed that if you go through the Gate you’re going to make a contribution and support the troops, if not actually fight. Demanding cash for saving people on the spot will NOT make you popular.
I mean, you could turn up with all the useful bits (artisans oil, potions, bandages), and give out business cards along with your wares. “If you have used my items, please come by my camp and make a contribution…”
As for looting off the monsters…
Due to a confusion issue…
[anything looted off the monsters could be an important piece of plot, including lucky charms, pet mice, letters home, and maybe even filthy handkerchiefs. It’s a shams, I wrote some nice letters home…]
… you won’t get much off most orcs (or indeed, other opponents, but it’s usually orcs).
Some orc healers or mages may be carrying crystal mana or herbs, a few important Elite Military Units may have one or two magical items between a dozen of them, and the big boss warorc is likely carrying a couple of enchanted items. And as mentioned, they’ll be tough, so looting will be difficult.
…20 Illium, or value equivalent, I think we spotted once when we were trying to catch an orc treasure chest carrying a lot of loot back to their lines…
As wraith33 says, it’s possible to get some pretty good kit reasonably cheaply. What sort of budget have you, and which nation?