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I've been trying to understand and reduce my impact on the environment, and particularly to understand what a really sustainable lifestyle would look like. I (roughly) get the concepts of ecological footprint and planetary boundaries.

But one of my difficulties is how to account for the impact of buying secondhand goods. I tend now to buy nearly everything from charity shops, eBay, etc and buy very little new "stuff". A simplistic viewpoint is that these have zero impact (other than shipping and other overheads). But that seems like a bit of a cheat. In particular it fails the test of "would it work if everyone lived like this" since it relies on someone else having bought things new. A bit like countries offshoring all their carbon emissions and claiming to have reduced their footprint.

Any ideas for how to do this in a practical way?

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    There is the question of when does it become second-hand enough to count? Suppose someone buys a new car every year, then sells it and buys another one, and so on, compared to someone who buys a new car and drives it for 20 years. If you buy a 20-year old car it's a lot more second-hand than one which has changed hands 4 times in 5 years. – RedSonja May 8 '15 at 11:44
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    Anyway, new stuff lasts longer - if you buy a new x and it lasts 10 years, is it worse in any way than buying a 5 year-old x and using it for 5 years? Only if used x don't land on landfill before they have reached their 10 years. This implies that you are indeed being sustainable when you buy used stuff, but only if we assume it would otherwise go to landfill. – RedSonja May 8 '15 at 11:59
  • Maybe the most useful thing to do is to keep your x, however old it is, or was when you bought it, repaired and maintained so it lasts long past its official die-by date. I read somewhere the Finns will despise anyone who buys a new car, for being too stupid to keep the old one running. – RedSonja May 8 '15 at 12:07
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The market value of the goods is often indicative of the remaining useful life of the goods. If you know the environmental footprint of the product when new, it seems reasonable to assume that a purchaser of the goods (second hand) is to be responsible for a share of that impact based on the proportion of useful life remaining.

For example, if there exists a car that cost $10,000 (when new) with a footprint of 5,000 environmental-harm units (whatever they may be; this is just an example. This works out to be 1 unit per $2), and you then purchase it for say $6,000 then you should consider yourself responsible for 3,000 units of environment-harm (ie still 1 unit per $2).

Of course, if you treat the item well and onsell it for $5,000 then you could consider that you've reduced your impact.

This kind of simple assumption is never going to be completely accurate, especially with factors such as government subsidies of environmentally damaging activities, but it's a starting point.

This idea holds up well in some key ways:

  • An item taken that was otherwise heading to landfill could be considered as having a zero dollar value, and if that's accurate then it's reasonable to assume you aren't responsible for any of its impact of manufacture.
  • The higher the price you pay for a second hand item, the more likely it will be that the seller replaces the item and causes additional environmental impact. It's therefore reasonable to consider that when you choose to take up their offer at the given price, you are responsible for additional environmental impact.
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What you are asking is generally referred to as the 'allocation problem'. There are a number of allocation methods that are often used in assessing the environmental impact of a product or service, but each allocation method has it pros and cons. Since the chosen method can have a big influence on the outcome of a study, allocation is an important and even controversial topic as experts have different opinions on which allocation method is best in which situation.

Some examples of often used allocation methods:

  • portion of mass reused: relatively easy to determine for physical products, leads to low impacts, if you reuse the entire product the impact is 0 (as you mentioned in your question)

  • portion of (embodied) energy: similar to mass reused, but may be more difficult to determine

  • market value: this is nicely explained in Highly Irregular's answer, value may be difficult to determine correctly, prices can be volatile

  • no/avoided allocation: may not be very realistic, leads to higher impacts, in your case you have the full impact.

Which method is chosen in practice depends on various things; the type of product, what environmental impact you want to measure, what data is available to you. Personally, in your case I'd go with market value.

  • I wanted to link this to en.wikipedia.org/wiki/Allocation_problem, but that does not exist while Resource allocation problem redirects to resource allocation. I added a note to the talk page. The Life cycle analysis article also mentions the problem of allocating impact between processes with shared flows. – PJTraill May 16 '18 at 19:23

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