There will be no selling of STEEM (see other reply below).

Next time the market pumps, sbd will have a limited supply and pump even harder.

You are right about this. The white paper suggests (and I agree) that when the SBD supply is relatively low, paying interest should be used to support the price of SBD rather than converting/destroying it. I agree with that. The advantage of using interest is that unlike conversions (currently; there is a proposal to allow conversions in both direction) it is fully reversible. The SBD then still exists later when the price may tend to increase, Interest can then be lowered or removed, hopefully weakening it.

In another thread there was a suggestion made that burnpost would just hold the SBD when SBD is weak instead of converting or burning it, for similar reasons. I have mixed feelings about that, but may consider it.