A seller asking for all cash would not be a red flag at all for me. I've never done a big sale with a person suffering from chrometophobia . . . yet, but I could work through it.
I do prefer all cash but I understand some people fear cash.
If a buyer shows up to buy my trailer with $5,000 cash and an offer to do the balance with a bank wire transfer, I'll work with that. If a buyer insists on full payment with wire transfer and no down payment, I'll try to work with them. I keep an account in a second bank open for wire transfers. When the transfer clears, I withdraw the cash and hand carry it three blocks to my primary bank and deposit it. I treat everything except cash like a personal check. Delivery is simply deferred until funds can be converted to cash and hand carried to a second bank.