"How can I sell my idea without it being stolen?" is a catch 22 type of problem. People wouldn't pay for something without knowing what they are buying. On the other hand, If you reveal the idea first, they won't pay for what they already know.
Some problems that seem unsolvable become solvable years/decades later when new developments in unrelated fields bring about the right conditions. Being able to sell ideas is one such problem.
The necessary conditions are (almost) in place for idea selling to become viable. These are the prerequisites:
Share to build leverage mentality: It is now common in society for creators to share their best work free of charge until it eventually becomes appreciated (think influencers) to the point that people subscribe to the value the creator provides. Subscribers (personal audience) provide leverage that the creator can use to their advantage. Be it upselling, soliciting support via donations, sponsorships, paid influencer marketing, paid talks, etc. The creator shares their work to build an audience and uses the audience to make a living.
Open ideation platform: Existence of an ideation platform like brainstorming.com that solely revolves around open collaborative ideation and idea-sharing. This provides a place for ideator-influencers to emerge and gain exposure based on the quality of their ideas. The only thing currently missing is the network effect. The platform needs to grow in popularity to the point where it becomes the go-to place for open ideation.
Top 10% of ideas inside a paid members area: Think Patreon or the Medium's business model where people pay a monthly membership to access a special area where the top 10% of everyone's best ideas are posted. For an ideator to have their idea included in the paid members area it has to be amazing (reviewed/accepted). The ideator gets paid a share of the memberships for every paid member that views their idea (Medium model). This is pocket change per member, but it compounds quickly as the number of paying members increase.
Having your idea included in the members area effectively means you've sold it to the public for the best possible price that the public is willing to pay for it. It doesn't matter who views or "steals" it. You get paid based on the quality of your previous work (people wanting to see more of it) and how interesting your specific ideas are. Having your best ideas mingle with other creative people's best ideas increases your chances of getting paid. Everyone's cool ideas motivate people to join the members area.
A list of other things worthy of mentioning, in no particular order:
It doesn't matter if someone copies the entire members area database of ideas and pirats it elsewhere. Where it can be taken down for copyright infringement it would be taken care of swiftly. Where it can't it would serve as advertisment. The database would be so amazing that people seeing it would want to sign up just so they can get the most recent ideas on a daily basis.
People who pay to join the members area get their money's worth of value many times over. It doesn't matter whether they like the ideas of the person whom they followed into the members area - there is plenty more for them to see - all included in the membership.
Reviewers make sure that all the content is high signal, low noise. All the titles are realistic (no click bait), etc.
Ideas become obsolete and get removed once they are known to be implemented successfully
The quality of your freely shared ideas build interest in what you have to say so that people would want to see your top 10% of ideas that are posted in the paid members area
A marketplace for ideas is very interesting and could be compared to a marketplace for opinions which is currently the domain of Twitter. The output is not comparable, just the concept of a community that comments on others’ comments. Within the platform, there are sponsors who are paying for their tweet to feature in the user’s feed for advertising.
In a marketplace for ideas, there could be a space for a sponsored idea. The concept is that the sponsor either has an idea they need a to break the back of and is asking for the community of creators to help them solve the problems limiting their idea from progressing, or; the sponsor operates within a field of interest to creators and is advertising their products and services through a sponsored idea.
If a sponsor is trying to solve problems and wants to leverage the collective intellectual firepower of a team of creators, they could prescribe the idea, the intended output and the financial consideration they are prepared to pay for a successful outcome. Milestone payments could be agreed, so that momentum is generated and a series of small successes leading to the ultimate goal. The sponsored idea could be worked on as a private project as the sponsor may not want their identity disclosed or their competitors aware.