for every buyer there is a seller and vice-versa. the borkers, exchnge, clearing corporations acts as an intermidiatory\facilator between the buyers and sellers. nothing more.
now suppose the seller fails to deliver the share, for whatever reasons may be, the first question asrises how the buyer (who already paid the money as per settlement schedule) gets his money back (and also gets compensated for the oppurtunity loss).
to address the same the exchage initiates an auction, where the seller (who has defaulted to provide the shares which he sold earlier) is bound to buy the shares back. in addition he pays an penalty, (5% in case if the shares belonging to EQ series). the whole proceeds, including the penalty is received by the buyer.
hope it clarifies.
cheers,
jdm.
p.s. - there has been thread on auctions, pls refer those for details