STEEM is a cryptocurrency and has a value. So it wouldn't be a big difference to sell things for STEEM than with FIAT. The main difference is the transparency. I wouldn't like it that everybody can see what I bought. So a shopping platform on Steemit should implement the encryption of memos.

But maybe it'll be easier to use SBD for shop systems on Steemit ? If they get 1 SBD pegged to 1 USD it will be a stable cryptocurrency and by this far easier for companies as they don't need to worry about loosing money with a moving chart.