For any blockchain, adoption is crucial to token value, but also longevity and health of the blockchain itself. Steem Marketplace aims to develop and sustain a marketplace platform that spans across a plethora of commerce options.
Commerce is an important step to more mainstream adoption, if you can get non-technology focused eyeballs who want to transact, that gives the underlying blockchain and its associated tokens real value.
When it comes to transaction based infrastructure, speed and security are the two biggest factors. You don't want users standing around for minutes while their transactions are confirmed.
Steem's three-second block times mean by the time a user hits "buy" or "withdraw" things happen right after.
Buy and sell domain names.
Buy and sell services.
Buy and sell handmade items.
Buy and sell apps.
Bid-based buy and sell.