[diego]

Perhaps it's difficult because it's inherently hard to completely transfer ownership of an idea. Suppose you have a great idea and you have a track record of great ideas to back it up. You offer to sell me your idea. Maybe the idea is the key to solving a famous mathematical problem or a novel approach to cure some disease. Problems:

1 - How do I know that you have a great idea if you don't tell me the idea first, in which case you can't sell it anymore.

2 - If you "sell" me the idea you still have it, and you can sell it to others (just like software).

That doesn't mean that ideas are worthless. It is possible to have an 'aha!' insight that is extremely valuable to you. There are lots of things that are extremely valuable to a person (meaning that you would pay a lot to keep them if you had to) but completely worthless others because they can't use them.

[davidw]

Yes, that's more or less what I said in the linked posting: ideas are non-excludable goods. To some degree you are right that they aren't rivalrous like physical objects are, but they do lose their value when they are widespread. A social-news-voting site isn't that hot an idea any more. It was a few years ago.

[13ren]

1. NDA or trust

2. exclusivity clause

One role of patents is they conceptually wrap up an idea, so you have a "thing" to sell (know-how must be supplied too - you're hired).