Selling per se is not a problem. The key is to have an equilibrium between the quantity of steem being sold and that being bought. Think of a bank: if everybody comes to ask for their money at the same time, the bank goes bankrupt. Steem is the same: if we all sell at the same time, the price plummets and confidence with it and selling begets selling and the whole "value convention" collapses.
So in a sense the key is "timing" - decreasing the "velocity" of STEEM (a term from monetary theory)