Okay, so I'm a little embarrassed. I clearly should have cited this before.
But better late than never. Michael Sandel makes an argument about when exchanges are not really voluntary:
The first objection [to the claim that exchange is voluntary] is an argument from coercion. It points to the injustice that can arise when people buy and sell things under conditions of severe inequality or dire economic necessity. According to this objection, market exchanges are not necessarily as voluntary as market enthusiasts suggest. A peasant may agree to sell his kidney or cornea in order to feed his starving family, but his agreement is not truly voluntary. He is coerced, in effect, by the necessities of his situation.
Wow. "Coerced by the necessities of his situation" sounds a lot like condition #6 here at the top right column of this blog, doesn't it. Awkward... Still, I'd like to think that the "disparities in BATNAs" definition gives it a bit more precision.
The second objection is to "corruption," where buying and selling changes human relationships in bad ways. (If you tell your spouse, "Hey, honey, you were great in bed last night! Here's an extra $20!" then that is not really good for the relationship.) (An extra $200,000, instead of $20, might create considerable enthusiasm, but it would not be authentic and would still corrupt the underlying relationship.) Some things can be commodoties and others cannot.
ATSRTWT, by all means.