Tax Lien certificates. Basically, you're giving an extension to someone who is delinquent on their property taxes, and ensuring that the local government, who probably very much needs predictable funds, collects them in a timely manner.
Some of these are cheap, in the hundred dollar range, which makes it easier to get started even if you don't have a lot of money to invest. Terms and availability depend on the area you buy them from. Interest rates can be very high, around 20% in some areas. In some cases (likely foreclosures), you can have a good chance of becoming the owner (or part owner) of the property, which can be massively profitable (but also a hassle).
On the other hand, some property is not that valuable, so you need to do some research. The lack of secondary markets for these makes them rather hard to sell early. And if you don't live in an area that offers good terms, you may have to travel to find the good deals, which is an expense. Some counties do offer auctions online, but you'd still need to do some research on the property.
Inferential distance? Or simply knowledge distance.
You lose me at "With portfolio margin". You're talking about financial instruments that, so I understand, you have a lot of professional experience in using, but I know nothing about these things. I googled "box spread financing", and it turns out to be a complicated instrument involving four separate options that, I'm still not sure what the purpose is. No criticism of yourself intended, but if a complete stranger started talking to me about box spread financing, despite it being a real thing I'd assume they were touting a scam. I don't know what "withdrawing excess "equity" from my margin account" means, nor the quote from Goldman Sachs (which would not come to my attention anyway).
And personally, I'm in the UK and a lot of what you're talking about is US-specific, but I can't even tell which parts are and which aren't. CD? FDIC? I do not know of a UK bank offering more than derisory interest on a savings account (typically 0.01% for instant access, 0.35% if you never withdraw money), but perhaps the banks I know of (retail banks) are not the sort of banks you're talking about. The Wikipedia page for Goldman Sachs suggests it is not involved in retail banking.