In some comments to Is it illegal to cut out a face from the newspaper? it came up whether you could legally buy an oil painting, add some stuff to it, and then sell it. I found that a provocative question, since it seems like the first sale doctrine allows it, but also you'd be unable to protect your own creative work in the action.
I then had a thought. This could be a workaround for derivative works. Keeping in print media, suppose I purchased many copies of prints and cut and modified them. Could I distribute the resultant works, since each came from legally obtained copies? Moving into the digital sphere, does this have the same effect? If I modify a film and have a legally obtained copy of the original for every copy I distribute, is there actually a copyright violation? The first sale doctrine seems to apply only to the actual physical media that your copy came on, thus the necessary copying nature of digital media means this won't work. Is that right?
A following comment in that question argued that the whole concept falls apart because copyright includes the rights to derivative works, which seems a pretty tight retort. I'm mostly interested in US law, but international differences are welcome.