There is no SBD entering the system right now from posts. It was suspended by the blockchain due to there being too much of it. There is some entering though SPS funding, but that stays in the SPS treasury until there are approved proposals. Right now there are a few approved proposals but not that much approved spending overall; most of the SPS funding SBD just stays in the treasury (and off the market).

The @sbdpotato posts are doing exactly what you suggest: buying what is already on the market.

With only 65k steem you can buy the supply on Bittrex and bring the price to 1 USD.

Such big market buys don't more the price on a sustained basis. They create price spike and then it goes back down.