While I can’t give specific overall numbers from Epic’s side, people selling content on the Marketplace are free to share whatever they’re comfortable with. There aren’t any rules against content creators sharing that information, so if you’re comfortable with it, you’re welcome to do so.
There are a handful of people that have found enough success to quit their fulltime jobs to sell content on the Marketplace and elsewhere, but it’s still too early to see this being a widespread, common result. One of the aspects of the Marketplace that I emphasize is that while we are young (just turned 1yr!) and don’t have the sales volume that other places might, Epic Games has a reputation for quality and a broad reach as a media platform, and getting your content accepted on the Marketplace is not particularly easy because of our high quality bar and requirement that content creators continuously update their content and support their customers.
I encourage people to use the Marketplace not only as a way to sell products, but also as a means of publicizing their content and services. It can help drive attention toward the game you’re developing (like W3 Studios does), or potential business toward their company (like T-Pose and Bumping Pub), or availability for contract or full-time work (many people). I like working with people to help promote their ability and content as I can, and in the coming months with new publishing tools we’ll be giving sellers, we’ll have more time and ability to do more community showcases and promotions like we did last week with PurePolygons (Marketplace Spotlight: PurePolygons - Unreal Engine). And as always, we’re open to hearing ideas on how we can help promote people and their work.