In magic, what you are paying mostly for is the secret to the effect, the price of the media which that is delivered on is really just a production cost.
Leaving all ethical discussion about whether is it fair or honest to sell someone else's idea aside. For a large quantity of effects out there, especially the ones you will purchase from E or T11, they are copyright protected, and that means the content of the DVD or book you purchased is protected, not just the specific media you have. If you were to say, purchase Window by David Stone, and then make a video explaining how to do it, then sell it. I can almost assure you that his lawyers will find you, and sue you. However things like Double Lifts, Top Changes, the Pass, I think most of these fall under public domain. If you purchased a DVD that teaches these, and you choose to produce your own video teaching these moves, you likely won't run into too much trouble, but keep in mind there are a variety of people who have taught it before you, so your video better be good.
Whatever the effect is, if you plan on making a video and selling it, do your research first, make sure you have permission to publish and profit from the effect. Purchasing an effect does not count as having permission to publish it, and especially does not count as having permission to make money off of it.