I have a number of photographs from a recent client shoot that they don't wish to use. Could these be sold by myself as stock photography? For that matter, would it be considered to be ethical to do so?
As for the legal issues, the crux of the matter is the model release. All of the reputable micro-stock websites that I've looked into (Shutter Stock, iStockPhoto, Fotolia, etc.) are going to require a model release if your client is in any way recognizable. Traditional stock agencies will have similar requirements. Even if your existing contract includes a line regarding re-use of images (i.e. in your portfolio, on your website), I doubt that a stock company would accept this kind of document as a "release." If the client does sign a model release, then they have explicitly given you permission to use the photos and you should have no legal problems in re-using the photos.
One potential legal issue, though, could come up with respect to the definition of "consideration received." This is a standard phrased used in the beginning of most model releases, because for the contract to be valid the "model" (in this case your client) must receive something of value from you in return for your rights to re-sell the images. If the client is paying you to take his/her pictures, then you may have a tough time selling to a court that the client received anything of value. Usually this would be something like money (if you pay a model) or photographic services (if you do a trade for shoot with a model).
Ethically, I agree with most of the other answers that you should pass it by the client or at least mention the fact that you'll be re-using images for stock photography. Just because a client isn't going to use the photo doesn't mean it's a bad photo or represents the person in a bad light. As long as you sell "reject" photos that aren't embarrassing, compromising, or the like, then I wouldn't have any ethical problems with it.
Check your contract. If ambiguous, talk to the client. If uncertain, ask a lawyer (or just forget about it, as that lawyer will likely cost you more than you'd ever get from selling the shots).
That's the most definite advise you can ever get here, as we haven't seen you contract, don't know the law where you live, where the client lives, and where the shots were taken (all of which might be of influence), and we wouldn't want to become legally liable if we told you something that made you do something in breach of the law or your contracts.
In the very least, you'd need a model release from the client, if they appeared in the photo. Possibly also a location release.
If the client is important to you, consider whether you would do harm to the relationship by attempting to sell the images through another channel. While some clients would not mind - after all they understand that you need to have an income - others might feel that you're double dipping.
Definitely ask the client, not a lawyer. In otherwords, the lawyer can say it is legal, but it does not make it ethical, or appropriate.
For future, you might want to add this to your initial contract, as an optional clause that the client can ok, if you sense that a stock opportunity might present itself during the shoot.
NO! it would be unethical if not illegal. Your clients would certainly not want picture, which they thought were bad, to end up on a billboard or milk carton or a powerpoint presentation. Even if you have a sound contract, make sure that you tell your client where the pictures could end up.. you don't really want bad publicity.
Check out http://www.theblaze.com/stories/mother-outraged-after-her-daughters-image-appears-on-pro-life-billboard/ to see the full extent of the worse that could happen..
Yes, you can, if your client has agreed to it upfront. Since you're asking, I assume your current contract is vague about that. Bringing it up afterwards is probably not worth the hassle and selling them without client's consent is subject to ethical and legal mess that could cost you more (in time, reputation or money) than potential earnings.
So, my advice would be to include the option in your future contracts and spend time making new photos instead of trying to crank out more from your old shoots.
Do you own the copyright to these images? Were you commissioned to do the work or not? That's the real question. If they commissioned and paid you to do the work upfront, then they may own the copyright. Just because they choose not to use them doesn't change that.
Even if you do own the copyright, if the images are clearly identifiable with the client (their products or logos for instance) then you could be on thin ice as well.