If a land can only be played when the stack is empty, I assume that means that an opponent cannot (normally) play a land in response to an ability? So if I had Limited Resources, Zuran Orb, and Land Tax in play on my side and both my opponent and I each had 5 lands in play, when they signal the end of their second main phase, I could sacrifice one of my lands to Zuran Orb without affording them an opportunity to play a land during the remainder of their turn?
As long as you sac your land in the end step, they do not get an opportunity to play a land since they can only do so in their main phase. If you sac your land in their main phase, they can play a land once the ability resolves since it is still their main phase. So, it comes down to when you activate the ability. And, to be clear, really has nothing to do with responding to an ability to play a land.
OK, so my opponent doesn't "signal" the end of his second main phase, he simply passes on an empty stack and when he does, if I sacrificed a land to Zuran Orb, he could play a land after the stack resolved?
Well, again, you get to control when you perform *your* action. If your opponent says "pass the turn" or "move to the end step" while they are in their second Main Phase, you can either act in their second main phase if you explicitly declare you are acting then. Otherwise, it is assumed you are acting in their end step. In either case, they get priority back after your ability resolves but if it is their end step, they can't play a land.
I question the bit about not signaling the end of their main phase though. I don't really know what you mean. Are you saying it is their main phase and, instead of saying "move the end step" they actually say "I pass priority"? The end result is exactly the same for both but the latter is exceptionally rare and players don't often move through the game like that. If you want to be sure of when you are acting in the latter case, you can just say "in your end step....".
I was a bit off with the mechanic of transitioning from one step to the next. I thought that when my opponent had nothing left to do during a step (e.g. second main phase) and the stack was empty that when he passed, the only cards he could play during the remainder of the step were responses to anything I did (i.e. things that went on the stack). I didn't realize that if I played or card or activated an ability that once that resolved he could continue doing things in the step such as a playing a land, summoning a creature etc. and the transition to the next step happened only when we both passed on an empty stack.