WWUD?

Let's construct a totally implausible scenario, just for the sake of discussion.

In it, you get your eggs from a local poultry farmer.  You really like those eggs -- the yolks are yellower, the flavor's fuller, the whites whip up meringuier, the shells are more substantial, you don't worry  about using them raw because you know where they've been.  There's only one poultry farm in the world raising hens that produce eggs for which all the above is true, and -- here comes the really implausible part -- you're its only customer.

Due to a combination of circumstances unlikely ever to recur, the farm has fallen on hard times.  The hens are getting old, egg production has effectively stopped, there's only one dozen eggs left.  Leaving aside the question of whether the eggs are fertilized or not (this is, after all, an implausible scenario), do you purchase and consume that last dozen eggs, or do you leave them to the farmer and hope a new flock of laying hens will be forthcoming?  Do you satisfy your immediate demand for eggs, knowing the long-term consequences for both the farmer and you?

Or, for our vegan friends, a similarly implausible situation:  A local farmer is the only producer of a uniquely flavorful and nutritious strain of corn and only a dozen ears are left.  Again, you're the only customer.  Do you purchase and eat them, or leave them as seed for the next growing season?

I like posing stupid questions like these to students, because as a rule the "correct" answer is immediately forthcoming.  Don't kill the goose (or the hen) that lays the golden eggs.  Don't eat the seed corn.  Don't be an idiot.

Some of the brighter students look for a way out of the obvious answer.  They ask questions like what other sources of food are available, how hungry they (as the customer) are, how it is that this farmer (whichever farmer we happen to be talking about) will have enough food to live on until the new crop comes in, so that the eat-or-retain decision obtains solely to the customer?  But the fact that they're trying to find a way around the obvious answer clearly implies that they understand what the obvious answer is; the silly scenario has served its purpose.

The follow-on scenario -- the one which starts to require thought -- has the farmer (poultry or grain, doesn't matter) located at a bit of distance from the customer so that there's no personal relationship involved.  Now your only concern is about consequences for yourself -- you don't know nor care about the farmer.  What do you do?

Then I add more customers to the scenario, so that while a decision to consume still precludes the possibility of future production, a decision not to consume -- not to purchase -- doesn't guarantee a significant chance that production will be able to continue.  It's highly likely that even if you don't consume the seed corn or the last dozen eggs, some other customer (one less responsible than yourself) will; future production will still be precluded, and you will have foregone that last bit of eggy (or corny) enjoyment for naught.

In real-world purchase decisions, of course, the situation is never so simple nor so stark.  You might be buying the last one that will ever exist of some product but it's somewhat unlikely that either you or the seller will know that fact.  More often, your consumption implies an infinitesimal degradation in the circumstances and conditions of future production, nothing like a total cessation.  But what's the cumulative effect of myriad infinitesimal degradations?  Even if they're on the other side of the world and experienced directly only by people you'll never meet?  Even if they're moderated, to some degree, by price rise due to supply and demand?  Even if acceptable substitute products are available in the marketplace?

By starting from an over-simplistic scenario, I find I can highlight concepts and raise questions that the prevailing consumption paradigm (purposely?) obscures.  For a lot of students, it's the first time they've been challenged to think about the implications -- for themselves and for others -- of the purchases they make.  By itself, it probably won't make much of a difference in their future consumption patterns, but it might be a start.