From a customer perspective they are the flea market and the Walmart next to the flea market and a bunch of stalls inside the flea market.
There’s no reason why they can’t vet all participants and stand behind their products. It’s not like it’s some onerous process. Walmart does it. They just have to pay out when something harms a toddler.
Walmart has 75M SKUs. Amazon has 350M. For many products Amazon never sees one - it only provides a webfront for vendors.
Would WordPress be liable for a vendor products using them as a web store?
I’m not sure what the difference is between selling and using something to sell. Amazon sells through Amazon.com, even though they allow third parties. I don’t think Wordpress sells under a single storefront.
Maybe the better question is whether eBay is liable when someone sells a defective item.