They also cost the time you put into farming them.

*shrug* In absolute terms, "use or sell" balances the simple "time to gather" vs profit from sale. In "use or (sell now, buy later)", the time cost is a constant and pre-paid at the point you make your decision.

Your point is well taken If you are gathering gold while leveling, as "time to gather" cost goes down significantly when you a) outlevel surrounding mobs and/or b) gain faster movement (mount, epic land, flight, epic flight).

The associated cost of "sell now, gather again later" during leveling is that you can't make use of the skill-to-be-leveled-later while you are leveling. And that is as intangible/subjective as the gathering cost itself.

@Eirik - I think he's talking about opportunity cost, regarding which I believe his point and his definition are correct.

@Yitzak: yep, we both are. I just framed it differently.

I think what Eirik brings up is this: What is the higher level skill worth to you?

If the skillups are worth more for you than the gold, for whatever reason, then it may be worth the opportunity cost to farm mats for consumption rather than sale.

This would be delayed gratification, and would be why one might go on to college instead of the work world, or why one might save rather than spend.

I think this is often overlooked in discussions of WoW economics, though it is something which needs to be considered on an individual basis.