1) No, these assets are not under the MIT license nor the CC0 license, but under our own royalty-free license as explained in the original post, read the full text here.
2) No, you are not allowed to sell derivative work (such as weapon models, weapon packs, or modified meshes from our blocks) in online marketplaces, these assets are meant to be used in a transformative way such as in your game, video or educational material.
3) We are offering these assets with a pay-what-you-want model to allow everyone to use them and allow those who can afford it to support our work. So you can get them for free, or you can pay, it's your choice. The license will remain the same regardless of the amount you choose to pay.