question Why domain marketplaces called "Domain Aftermarket" "Secondary Marketplace"?

Because they are selling pre-registered domains.

Further to what @stub said - on these platforms, you are bidding to receive a domain that is already registered right now. The distinction being that you are going to bid to receive a domain name that has no period of non-registration between it's current owner and yourself unlike a hand registration of a domain name which is not currently registered.