A lot of feedback here mentions tax compliance. Perhaps you could use a company like paddle.com to actually handle the payments and act as a merchant of record for each transaction. This way you wouldn't need to worry about calculating taxes and collecting payments from the end customers, you would still need to handle distributing income to sellers. I'm not sure though if Paddle supports selling such 'umbrella' type of products.

Anyway, the idea is great, I view Paddle as an order of magnitude less work for the seller compared to lower level services such as Stripe, and your service could be an order of magnitude less work than Paddle for products that fit such simple selling model well.