I imagine the biggest challenge is some kind of central payment authority and dispute management. I won’t touch anything to do with Meta, and as a former 6 figure a year eBay seller, the total percentages and final margin for the seller are just not viable. Between tax, shipping, and fees I averaged 35% overhead with a 100% positive feedback. With a keystone product, that just doesn’t work. This should be more like 20% to make a sustainable business. I have no clue how anyone would build a more effective dispute and payment system but from my experience on eBay, around 5% of all customers are scammers.