What is the ruling on subscribing to a platform that imposes financial penalties and other sanctions on the seller if he violates specific conditions related to the quality of goods, their presentation, and his commitment to deadlines?

The penalty clause in rights and obligations—excluding debt—for compensation for actual damages incurred is permissible. If the financial penalties imposed by the platform fall under this category, and are taken commensurate with the actual damage or to compensate for deficiency and the like, then there is no harm in that.