Good question, @baah and thanks!

For transactions using STEEM/SBD; the built in escrow feature will be utilized. For established merchants, funds will be release upon tracking info availability. For new merchants, funds will be held for 3 days post tracking info availability and then released. For transactions paid via Paypal or Visa, scams can be resourced directly to them for resolution.

For antiscam actions, we will first profile out any users contained in the Global Black list. Second, any IP from known suspicious sources will be blocked.

For outright scams, our function will be to investigate, mediate and block proven scammers.

The buyer/merchant will each also be rated per number of successful transaction and feedback scoring. Known scammers will obviously be at the bottom of such a scale. Sellers can also be able to block out unwanted buyers, but this feature will be a Phase II.

At the end of the day, reputation score will play the biggest role. In fact, the reputation score of buyer and seller will have calculation component in the distribution of reward tokens.

At the end of the day, we are a match maker of willing buyers and sellers. We admit, that all scams won't be detectable nor preventable. So, the scenario you profile can occur. Merchant/Seller reputation scores will have the strongest preventative role.

In other words, if you're buying from a low scoring merchant....the risk is pretty much on you and vice versa for seller. The seller can block out a threshold level of low rep scoring buyers.

Yes, on launch, all will have same starting rep scores. However, the growth process will enable the development of established sellers and reputable buyers.
One of our goals is to populate the marketplace with established Merchants from ebay, Etsy, Shopify as this would mitigate the "risk" perception.