If you take open source software or some white label solution and sell it, you're a founder.
If a company goes bankrupt, you buy the assets and then launch a new company. Founder.
If they already had a general release of product or service. I wouldn't consider them a founder if they joined afterwards.
In my opinion Instagram sold a bit early[0], but coming from an app that existed merely as the first widely used iteration of phone-camera filters, I can't necessarily blame them. Who'd have thought that Instagram, and also Giphy to an extent had -any- form of business model or potential for profit? I sure didn't.
[0] - sidenote: (in the financial and growth sense, not the chronological sense)
I think you have heavily heavily discounted that the people building aren't the same people ready to make the same time and financial risks.
That site is doing two things: selling projects, and finding cofounders. These are two separate functions if it isn't clear. You can buy a project and find a co-founder, not buy into a project and be the co-founder with someone that has another job.
Yes. I'm always interested in cool security projects.
More broadly is there a market for side projects? I could understand someone's side project expanding to the point they need to bring someone else on board (which is what the name implies to me). It seems to me that if the founder is selling its probably because there isn't money in it. Or at least I would be concerned that the bulk of sales are for that reason rather than because the founder has another job or something.