. What normally miners do with their mined bitcoins? They used to pay salaries in CNY, electricity... etc. How would the carry on after 30 September.
In all honesty, the increase in Bitcoin value would already have led to most miners having ROIed, it isn't that much of an issue considering the salary and electrical fees there.
. If not, how would they keep running their business?
Bitcoin is not illegal there. Exchanges are prohibited from operating but people can still trade Bitcoins within themselves. Its impossible to restrict it at all.
. If they stopped mining, what would this mean to bitcoin? Would the remaining miners be enough to keep the transactions going?
Yes. Bitcoin will continue as long as there is a single miner. If majority of the hashrates were to be shutdown at the same time, blocks would take an average twice as long and the difficulty will adjust to compensate for it by the next difficulty adjustment. Nothing is affected.
With this decline, won't the miners be selling more bitcoins to cover their costs?
I've seen a documentary by Vice and I vaguely remember that the owner of the farms said that they hold parts of it and sell off the others. Miners would sell more Bitcoins but it probably isn't that necessary.