That's what the clearinghouse is for. They are the counterparty for both sides. So the seller's counterparty is the clearinghouse, not the buyer, and the buyer's counterparty is also the clearinghouse. The clearinghouse requires broker margin accounts so that they can close out a trade when there's a failure. And the amount of collateral depends on risk.

(This is also why GME trading was halted by some brokers--assymetrical trades and increased volatility meant there was greater risk for trades in those stocks for those brokers, so the clearinghouse demanded more collateral from the broker. Broker doesn't have that collateral right away, they can't make the trade.)