This guy did it to land a job. Hopefully he's done with spec-work like this and his new employer makes sure to negotiate rates ahead of time for security reviews.
Since there is no honor among thieves every buyer has to assume that any bug they buy will also be sold to other buyers. With even a moderate bug bounty in place it becomes a prisoners dilemma for all parties who know of the bug. The first person to disclose the bug captures the bounty and the remaining parties get shut out.
Since everyone in the market has to assume that everyone else is cheating the market collapses. Microsoft has a paper on the economic incentives of the underground economy that covers the topic nicely:
https://www.microsoft.com/en-us/research/publication/nobody-...
https://www.google.com/about/appsecurity/reward-program/inde...
How come accounts.google.com more severe than others for XSS ?
Because that's where the jewels are: "Control, protect, and secure your account, all in one place"
We should connect, my company doesn't have anyone fully remote right now but maybe we could do in the near future...