I suggest checking out the topic on GameMaker Community forums, especially this post: https://forum.yoyogames.com/index.php?threads/amaze-me-game-jam.86376/post-51671...
Relevant bit:
> I had brought this up but I think we ultimately wanted the rules light as a bit of testing ground for how we are going to be running future events. [...] With the theme in mind you are welcome to toy away with ideas or even get started. Likewise if you can re-use previous assets to fit the theme you are welcome to do so! Obviously don't get stealing things - maybe we should make that clear...
While it doesn't directly state Marketplace assets are allowed, it does mention "previous assets" which are roughly in the same category (resources made before the Jam, whether by the participant or someone else).
Also, elsewhere in that thread I summarised a few points: https://forum.yoyogames.com/index.php?threads/amaze-me-game-jam.86376/post-51673...
In particular this:
> 2. Using pre-existing assets (be it graphics, audio or code frameworks) is perfectly acceptable, as long as there's no stealing (no unlicensed use).
Ross Manthorp (representing the organizers) gave this post a like and didn't add any reply that disagrees with me, so it's reasonable to think he agreed with what I wrote.
Hope this helps. ^^