This is a question for celebrity photographers. There are many online services now that market celebrity photos to media publishers. One example is the site WireImages. I am wondering if sites like this offer pay in advance options and sends its photographers to events, Or it buys from photographers and then resells to media publishers.
I'm sorry, I am not a full-time celebrity photographer but will try to help with an answer.
As stated on their web site, WireImages uses there own photographers. Other companies handle photos and photographers in other ways. Some services are going to focus on buying individual photos from photographers who they have relationships with and others are going to hire photographers to work for them full time.
I do not believe there to be a large market for celebrity photos taken by unknown photographers unless they are sensationalistic photos that have a unique story to tell. Services are going to work with people they know and shy away from managing input from hundreds of photographers that may or may not supply a steady stream of photos.
Just because there are "many services" doesn't mean they all have customers. A lot of them no doubt are dreamers hoping to make a ton of money quickly leeching off a business model they think must be successful because so many others are doing it (thus inflating the supply market artificially without any demand market).
Most tabloid publishing houses have their own vermin on staff full time whose sole job it is to stalk their targets 24/7 and come up with juicy photos and stories. Respectable publications usually rely on official photographs released by the celebrities or their agents, and/or have their staff visit official events and have them on call for covering emergencies. They might also do business with established stock photo agencies, who typically buy only from known professionals with an extensive portfolio. Those professionals will often be under contract to supply a steady stream of quality images to that agency.
For John Doe with his brand new $500 camera, they have no need.