[Kreij]

When one views a used item in the physical realm it usually means that some measure of the life expectancy of the item has lessened (it's used and has garnered some wear or in some way is inferior to a new version). This is why laws prevent people from reselling used items "as new".
Even if the item is returned unused, there is no way of determining if it has degraded.

In the digital world there is absolutely no difference between a used copy and a new one.
The used copy (and key) are every bit equal to a new one, and thus the used version looses no intrinsic value based on the fact it has been used. The concept of a "used" digital copy has no real meaning other than what might be arbitrarily applied to it.

Unless a digital copy of something is no longer available, it's just another copy.
People try to equate digital resales with resales of physical items, but you can't. The comparison has no meaning.