Hey.
I tried looking for it online, but couldn’t find any info.
So, suppose I bought some marketplace assets, started a project, and started building it.
Then, got other people to help continue with the development of the game.
Will they all need to buy each and every asset that I added to the game?
If so, under which circumstances?
This seems to be a little bit of a grey area, so I really want some input on it.
Thanks 
Take a look at this thread: https://answers.unrealengine/questions/208269/marketplace-content-in-a-team.html 
"The assets you purchase are tied to one account. Once added to a project you will be able to share that content with your team as long as it is a part of a project that you have the rights to.
Here is a link that goes over it in a little more detail, specifically the section “WHAT CAN USERS DO WITH MY CONTENT ONCE THEY PURCHASE AND DOWNLOAD IT”. If you have any other questions, feel free to ask."
Alright, thanks for the answer
Hi,
Im afraid those links dont work for me…they 404.
Unfortunately they are outdated.
New one: https://publish.unrealengine/faq