You can't sell food stamps so easily anymore. They used to be paper currency, but they are in the form of a debit card now, still possible to sell, just requires some planning

Every few weeks I get a person approaching me in the grocery store offering to buy me $2x of my groceries for an $x cash payment.

Exactly this. More food for less money is a good deal if you are just-over-the-line for getting food stamps or the ones you have don't go far enough to really feed your family.

To be fair, folks will do the same if they need cigarettes or need to fix a car to get to work and back.

I agree. Folks will find a way to get their needs met.

You don’t solve trauma with cash, but you also don’t make it worse and the benefits to others seem to be greater.

A regulated nervous system is a need, and if someone never developed that capability and doesn’t have awareness or resources to develop it as an adult, it’s actually a pretty logical move to use drugs.