(i was thinking advertisement plugs to promote posts could be paid for by outside currency.)
though an alternate currency would be nice to exist on its own without need to track any other one(s).
so then sale of goods in exchange for steem bucks would allow people who want to promote their content to do so using the bucks paid while not having to bring in any fiat paper money to the system nor ask users to acquire that currency to buy what they like.
a seller just brings his goods or services for trade,
in exchange for steem he spends to promote his goods or services available for trade for other goods or services being traded in the market or bought with whatever currency is used to negotiate the transaction. (currency is just a ledger system, dollars are only needed for paying government taxes, [digressive point to following up on the topic of paying dues, a service people could provide in exchange for officially produced steem {my ignorance about where steem comes from} is to provide cpu cycles like SETI@home or to do file organization or tag pruning {unless that is already automated} or they can collect fuel to deliver energy to the steem server farm or lay fiber or mirror the site to help absorb bandwidth costs in exchange for steem reimbursement to get them SEO preference by having promoted their fiber laying or energy producing company's logo and deals on the site. ie "today's steemit crypto-social networking service is brought to you by alan and sons wood gas supplies™ they're helping to Make America enerGy independent Again and are ready for your business. now accepting steem, bitcoin, doge coin, silver coin, gold coin and carrot coins by the pound as payment choices." and if the service was a membership dues charging site, like a members only trading post, albeit with publicly visible ads, then steem could be bought back by the system as it recycles itself. posts could cost money even grammar mistakes, refusal to use capitalization or obvious violations of decent use codes against profanity or grievous misrepresentation of fellow members can be fined in steem so that either such posts get removed and a waiting period elapses or they have to get enough upvotes to pay the fine before more posts can be made from the posting account.])