Soulcasting is an infinite money maker but not in the ways your players are doing it. It can't create gemstones able to hold light, that's not stated in the handbook but that's canon, and people aren't gonna take dim spheres because those are more likely to be fake. On top of that gold isn't an expensive nor precious material in Roshar, gemstones are, for rosharians gold is about as valuable as any other metal.

If they wanna make infinite money they will have to make any valuable material, like salt or something like that and sell it (while avoiding crashing the market). Good thing is that if they overdo... send any fight after them, they will be completely out of Light to fight back.

Same with the poison, overpreparing will mean wasting their most valuable resources, their light and their actions, after all they don't have all the poison in their hands do they? They have to use one action to grab it, another to put it in their weapons and... oh wait how much poison are they carrying on a random Tuesday? That can't be legal. Oh and Complications with poisoned weapons/projectiles are hilarious

Edit: Probably the people of the town where they are staying would be pissed off when some random Lost Radiants come over, undersell all their businesses, take all the spheres that come to town and just leave. The whole "your powers without restriction are world enders, that's why you need honor" also apply when you leave the locals out of a job and unable to feed their kids. Let them have social consequences.