Would you buy tech ideas with the initial research and documentation? If yes for how much?

Some people are great at coming up with viable ideas, others are more capable of executing them.

If you're the latter, it could make sense to buy a developed idea the same way it makes sense to hire a developer if you have an idea but no coding skills.

The issue, however, is that ideas morph over time (customer feedback, trial/error, etc) so be aware that if you buy an idea, you aren't buying a business in a box; Creativity is required every step of the way.