In my experience, on this topic, people are rude, stupid, or both. Often both. They will always try and "do the math" without any sense of the benefits they are receiving, that may not transfer to different situations. Like the discount you're offering for bulk purchases. They will also try to bargain in ways that don't always seem very respectful - it can be stressful to deal with, if you're not wired for that kind of stuff. (I am not.)
One of the things I've done to try to head that stuff off is to actually list what my prices would be if they didn't buy the whole batch:
Buy 1 for $150.00
Buy 2 for $270.00 ($135 each)
Buy 3 for $360.00 (120 each)
Buy all 4 for $400 ($100 each!)
This has the added benefit of emphasizing the deal they're getting (oooh look if I buy all four I'm saving...) It also creates a point of reference if they choose to try and haggle with you. They know what you expect for a single purchase.
I've had the same experience with time. If I'm offering a service, say charging $40.00 per hour, someone might contact me and say "can I buy 1/2 hour?" From their perspective, it may seem that a half hour should cost $20.00, since they're getting half of what would cost them $40. But I have to deal with all the scheduling, maybe travel, and any other headaches that come up, and that might not be worth $20.00 for me. So if I want to head that off I should probably write a listing like
1 hour $40
1/2 hour $25
(I actually often don't, because I don't really want to do half hour sessions. I make exceptions, but I don't want to encourage people to ask. If I was getting a whole lot of requests though, I'd probably include that price schedule in my ads.)
One funny experience I had was in dealing with low priced items. I don't really want or expect to get much for the item itself, but if I have to be here to meet the person, I don't want to do that for less than $10.00. So I've at times been on the verge of writing an ad like:
1 for $10.00
2 for $10.00 ($5.00 each)
3 for $10.00 (~$3.33 each)
or get all 4 for $10.00! ($2.50 each)
So basically, show up, give me $10.00, and take as many as you want.
You will still have morons who will look at the best deal and try to do the math ("uh, can I buy one for $2.50?") But I think it at least happens alot less.