[meric]

It was meant to be a humorous post, but I'm glad you humoured me anyway. Let's do this.

By your definition, it would seem I purchase my sleep by not working while unconscious, which is obviously ridiculous.

My definition of buying means giving up tangible goods, i.e. gold, money, shares, in return for a change in the world (e.g. receiving a massage).

So, to exchange time for sleep would not fit as your time is not a tangible good. And since you require sleep to work, it would be more accurate to say you earn the ability to work by sleeping. I stipulate no buying is involved.

Opportunity cost does not necessarily mean buying. Exchanging a tangible good for something else does.

I suppose where your definition and mine differ, is in your definition you require a sentient seller, and in mine the seller could be non-sentient, or could be metaphorically represented by a collective of gold owners who would benefit from you losing your gold to the ocean.

[lotsofmangos]

To buy something I definitely think requires a sale and an implicit contact between two parties. Also, it is perfectly possible to buy something with your time, tangibility is not required for something to be purchased, definability for the purposes of the contractual arrangement is the important bit.