The concept of multiple tags is confusing, it's a bit all over the place and the shopping experience on steemit is not good. There is also an escrow at the blockchain level so the escrow system you propose seems a bit backward.
I think @steemshop is a good initiative but the way you are doing it is not going to get much traction.

My advice if you want to work on a marketplace is to either build your own site (like you suggested) or code a simple market that can be integrated in steemit
For example there is the market page https://steemit.com/market we could have the https://steemit.com/shop page.