To sell shares without buying in day trading, in other words, is short selling. Short selling in the context of markets is all about selling stocks that you do not own (or which are not there in your demat account).

In the Indian context, short selling is only permitted for intraday purposes. If the stock sold is not covered by end of the day then it automatically goes into delivery selling and you need to give delivery of shares from your demat account by T+1 date or your order gets squares off by the software.

If the software does not square off your order, then it is presumed that you as an investor has to deliver shares from your portfolio or
if you do not have shares, the auction will take place and the difference of the cost has to be born by you ( shares bought minus the delivered to the buyer).