Upload & Sell: Off
If it's open to the public, in a public place or not some place or event that would suggest that there would be a reasonable expectation of privacy, then taking pictures is just not a legal problem. Nor is offering them for sale. This is like the news media attending and reporting on an event.
Event organizers might choose to incorporate some sort of model release in their registration materials if they should want to use images for promotional purposes.
Where there might need to be some contractual protections is if the promoters/organizers have entered into a contract to have the event photographed and they want to protect their own and the photographers fund-raising opportunities. Sporting events may have these arrangements. Typically the event controls access, controls uniforms or logos, etc., through copyright and trademark type protections.
It might be easier to tell if there might be a concern if you described the "public" event.