I think I've seen this subject before on the forums, but I'll go ahead and toss in my two cents anyways.
Before we even begin to discuss ownership we must first have an understanding of what is Public Domain, and what isn't. Public Domain is intellectual property that is either so old, or so broad that it can not be copyrighted by a single entity. Thus making it fair game for anyone to release a product using it. An example of this, I believe, is a Double Lift. You, and I could both release a tutorial on how to perform the Double Lift without being in danger of each others copyright. The only thing we couldn't do is copy each others presentation word for word, and motion for motion.
Next is the subject of revealing sleights online. From my experience exposure has a purely intrinsic definition. Meaning that it is only exposure if the sleight can be learned, or downloaded for free. Once you have to pay a price to learn it, it no longer is exposure, and becomes educational. While some cases of revealing or exposing sleights online may fall into a legal gray area, the morality of the ethics involved is black and white. You shouldn't ever expose magic online- period. Thus in my opinion, and I know others feel the same way, you step over the line the minute you reveal anything. While exposing something like the Double Lift may not be wrong legally, morally it is.
As for performing other magicians effects; this is another complicated subject. Technically when you purchase an effect, you purchase the rights to perform said effect. The effect usually comes with patter, and sometimes that patter is the best to use for that specific effect. This is often the case in mentalism where how you say it is just as important as what you say. Deviation from the provided script without an explicit understanding of how mentalism works could damage the effect more than help it. So in short, yes you perform others material providing you've paid for the product, and obtained it through legal methods.
While nothing legally prevents you from watching a demo video for a product that is available on the market, figuring out how it's done, and then performing it morally doing so is morally wrong. Practicing magic as a professional is more than just being an entertainer, it's also being respectful of the ethical standards upheld by our community. You shouldn't perform an effect you didn't pay for.
On a lesser note if you find yourself working the same venue as another magician then it is considered bad to perform the same effects as what he is. You should discuss with each other first to make sure that you aren't doing anything similar before the show starts. If there is a mutual effect being done between the two of you, find a compromise so that only one of you is doing said effect.
