Ok, I'm at a convention attending a lecture and the speaker demonstrates a prop and explains the working. This is something that he is selling at the convention. Am I ethically required to buy the prop from him in order to perform it or does the price I paid to attend the lecture give me the right to construct my own apparatus and use that?

I would say that if he demonstrates the method that you are free to build it for your own use, but not as a sub-manufacturer. If it is in fact a lecture, he is ostensibly teaching you something and you are allowed to go forth and use that knowledge.

The fact that he is selling a similar item does not prohibit you from making your own. If he did not explain the method, then he is really doing a dealer demo and expects you to purchase from him.

I agree, that makes the most sense. Many times they will explain the workings, then as a footnote, explain why the model they make is superior to what you might be able to make yourself. That's a great way for them to offer the prop for sale if you want to make it easy or just build it if you're so inclined.

I would assume it is OK to build your own. Did the speaker say anything to indicate that it wasn't OK?

Lecturers who demonstrate the workings of their 'inventions' as a lecture and don't allow you to build it, are in the same category of Magic book writers who teach material and limit performance rights...
in both cases they have taken money under false pretenses

Or beautiful women who take their clothes off in front of you and say don't touch me.

I agree... and it's sometimes "Don't touch me...unless the price is right" and with both this AND the magic case, you know what we call people like that.

Ruthless....

The lecturer didn't say we couldn't build our own but it seemed subtly implied to me. It seems you all are pretty much in agreement with me (except for the women thing) so I feel better about it. Thanks.

Went to a lecture once and the guy actually told us to save our money and go ahead and make it ourselves! I agree with the other posts - as long as you aren't going to mass produce and sell the effect you should be fine.

Oh yeah..."the Dude abides."