Stock purchases are settled after 2 days. If the buyer goes broke before that, the seller would be left holding to bag. To mitigate this risk you need to post collateral.

of course, but if you buy without margin the balance in your account should already be enough to cover the collateral.

But it's not the brokerage's money on the line, it's DTCC's. From the perspective of the DTCC, there's no difference between a retail brokerage buying $10M of GME and lehman brothers buying $10M of GME.

But if the brokerage already has your money they can use it to post collateral. And yet they banned everyone from buying. A lot of stuff doesn't add up here.