Just got a chance to read his post about a steemit marketplace. I agree.
This would be a great step in the right direction. As stated before, we wouldn't want the merchants to then go and cash out as well. Hopefully if people are buying with SBD, they will then reinvest to continue to have more in their account.
It would be great if the merchants had to pay a fee in steem or could pay for advertising similar to how you state the promotions tab works.