I'm trying to make sense of what you're saying but you might be confusing the concepts a bit.
Settlement and margin call are only relevant for people who have borrowed bitUSD with bts as collateral.
If the price of bts drops too low, there will be a margin call.
Settlement is a bit different. If someone has bitUSD (borrowed or not, it doesn't matter) and presses the force settlement button then he will be given bitshares no matter what 24 hours later.
So basically the people with the lowest collateral will lose their bitshares, NO MATTER HOW MUCH collateral they have.
This is the most dangerous and most important point about borrowing on bitshares. Don't ever be in the list of accounts with the least collateral no matter how safe you think you are.
Because even if you can never be margin called you can still be force settled.
And it's actually happening in huge numbers on the bitCNY market, but not that much on the bitUSD market.
Does this explanation make more sense?
Again if you haven't borrowed anything, then all of this doesn't apply to you, and maybe that's where your confusion about selling and buying is coming from?
I took a screen shot that show when the settlements are happening and at which price. Look at the bottom left corner.
It basically means bitUSD will be taken out of circulation when the settlement happens.