I've never seen a clear explanation as to whether first sale doctrine applies to digital media. Theoretically, one could sell one's purchased video clips and then immediately delete them from one's computer. If the clip was solely possessed by the new owner, it could technically pass first sale muster, as no duplication had occurred. However, there is an inherent copying involved in sending the file to them (unless, I suppose, you mailed the original hard drive to the recipient), so it probably doesn't pass first sale muster. Hard to say without physical media involved.
It's a tricky issue for sure. With a physical video, like a DVD for instance, at least you can sell the original. This doesn't apply with a clip since, as you say, the simple fact of sending it involves making an electronic copy. Then again, one could sell an original DVD, but only after making a copy for himself, possibly with no loss in picture quality. That would be illegal, but there's no way for the buyer to know.
It's even easier to trade or sell clips while keeping a copy for oneself. Sure, if one is really honest he could potentially delete it from his hard drive. But again, how can this be verified? No tangible original copy exists, so what keeps someone not only from making a copy for himself, but actually selling copies to multiple people? If anything, that's even worse than sharing a producer's clips for free, as one would make money doing it. Very lame.
Since it can be easy to make identical copies even of a physical video, trading/selling any copyrighted material is iffy at best. However, I tend to feel that if someone owns an original physical copy, he should be allowed to sell it if he wants. I don't like it very much, but he does own the copy, so it seems fair to me. As long as he doesn't make a copy to keep, of course.
However, due to their very nature, I don't think any sharing, trading or selling of clips should be allowed, no matter how honest one might be.