It really doesn’t matter that it’s a digital copy v. a physical one. Legally, the principle is the same. You should have the right to sell it. As long as they make a reasonable effort to make sure there is not a copy of it on the computer with a search program or something, that is adequate.
Even with physical CDs, there is no way to ensure you didn’t make a copy of the movie or CD prior to selling it.
There is simply no way to do so. There is no way to ensure you didn’t take a photo of every page of a book before you sold it.
The fact that it’s easier to make a copy with a completely digital copy is irrelevant to the legal principles.
Legally the principle is the same, but logistically it’s very different. If I sell a book I don’t have the book anymore, I may have copied it but copies lose quality. If I sell a CD I don’t have it anymore, I may have copied it, but again on some level I’ve lost quality if only on the cover and liner notes. If I sell an MP3 chances are I still have it, even if they’re buying software deletes it from my system I have other completely identical copies (guaranteed, I always have at least 2 versions of my MP3, the one I use, the backup and until I do my periodic cleanup the source).
It’s the copy fidelity that’s the problem. For things up until files copies lose some kind of fidelity, so if you sell your original but keep your backup you’re keeping an inferior product. An MP3 is an MP3 is an MP3, my backup is completely identical to the original, and in fact even in selling the original I’m really making another copy, I’m not selling those sectors on my HD, I’m selling that sequence of ones and zeros to your HD. It does matter to the legal principle, because now you’re selling copies not the original, and only the copyright holder is allowed to sell copies.