Do you believe in equivalent exchange?

Full Metal Alchemist anyone?

Equal or lesser value? If that's what it is, methinks I spot a fallacy: if Person A gives something of lesser value to receive something of greater value, there must also be a Person B who is giving that something of a *greater* value to receive that something of a *lesser* value. In that example Person B is not behaving according to your description of equivalent exchange.

So let's forget about the "or lesser" part and focus on the "equal" part. In a physics sense it makes sense to me; "every action must have an equal and opposite reaction" and all that. Maybe it doesn't always make sense at a quantum level, but at a quantum level not much does :P
I don't think you can apply it to exchanges between people though, because people aren't perfect at judging the worth of potential outcomes. Yesterday I paid £10 for a pasta bake that was very nice but not worth such a lofty price. The reason for that is that since no transaction exists in a vacuum, there are always extraneous factors skewing human perception of worth beyond the value of the item; I wanted that pasta bake because all my friends were eating and I didn't want to look like a weirdo, and because I expected it to be nicer than it was from the photo in the menu. It was nice, but not exactly £10 of niceness - thus the rule of equivalent exchange was broken.

Value is subjective. What has little value to me may have a greater value to someone else. We'll use my garden as an example. I have a bunch of tomatos. I can't possibly eat them all. I may gove them away for free, or trade them for some cucumers or other garden item. At the store the food may cost more or less than store bought food. But I place a higher value on garden fresh veggies than store bought even though they likely cost more at the store than they do at a farmers market. And my situation doesn't allow for much time to do yard work. But I will happily pay a friend in beer to do labor that may translate into $10-$20 an hour. The beer doesn't cost that much.

I too follow these values

What is one of my turd loaves worth?

In my head, I would like to believe that the world works like this, but it doesn't. Personally, whenever I really want something, I give something much more valuable instead; and almost always I get nothing back. On the other hand, I have gotten so many great things from people without even doing anything.

However, in the most literal way, there is some truth in this idea. Whenever you give something, you always receive something of equal value back, but that only works if you take "indifference" and "silence" as part of what you get back.