The message format would need to be defined, so I'll just make something up. This is the private marketplace version, where the market is visible to those who know the marketplace password.

There can be a FileMarket (selling of files with arbitrary content) and MailOrder (seller sends the product in the mail). I'll begin with the FileMarket because it is easier.

Each message contains a prefix, which is the first 4 bytes of the SHA-256 hash of the marketplace password. Message content is encrypted with AES-256 using the marketplace password, except for the accept message, which should be readable only by the buyer.

Seller declares he sells something by sending himself a message:

sell/product name/description/price/keyword1(optional)/keyword2(optional)/keyword3(optional)/image link (optional, image is encrypted with marketplace password)

Setting keywords allows buyers to find products by searching.

Buyer sends the seller a message to buy:

buy/product name

Seller sends the decryption key for the file to the buyer:

accept/product name/download link/key

If for some reason seller can't complete the transaction:

reject/product name

User sends feedback to the seller:

feedback/product name/score/text

Seller feedback and available products be will be visible on the blockchain to those who know the marketplace password.

EDIT:

Seller sends himself a message to provide general information to the buyers:

seller_info/text/allow other sellers (yes/no)

The first seller can prohibit other sellers from joining the marketplace.