Google directly competes with the grey market for vulnerabilities. They are competitive in a bunch of different directions:

* They pay for vulnerabilities without reliable exploits (more for vulnerabilities that are demonstrably reliable).

* They don't require you to actually build a reliable exploit chain.

* They pay up front, not in tranches.

* They work with essentially all comers, unlike the grey market, where you're generally subcontracting to sell your first few.