This is a really good idea! I've long been a proponent of an "eBay/Etsy/Craigslist" type peer to peer marketplace as a way to get Steem (the currency) to become something commercially viable... which (hopefully) will also get investors interested in Steem.
I am no expert but if there is a growing marketplace using exclusively steem and steem dollars as its currency, and this marketplace is only available on steemit, that should certainly get investors to at the least check it out :)
Part of the "key" to it is to make it a cool enough actual app that people from off-Steemit would look at it as a viable "additional sales venue/marketplace" for whatever they are doing... and join Steemit, just to be part of it... INDEPENDENTLY of having an interest in alt coins or an interest in blogging. In other words, let this be an "expansion slot" in the overall Steemit ecosystem.
I had not considered this aspect and you are right of course! Great thinking. Don't suppose you know any awesome app makers??? At least we are on the right platform to find one :)