How do you go to the bank for a cashiers check on Sunday?
What people have agreed to provide you with $20k in cash? (Personally I'd tell you to pound sand if that was your only answer as a seller. If you've convinced someone to do it, then more power to ya)
How do you go to their bank and get cash when they are out of town or don't have a local bank? (My bank is USAA, no means to withdraw large cash sums locally...)
All your suggestions are great, but also assume that you the seller are 100% honest and only protecting yourself at expense of buyer. Real life requires said buyer to be just as cautious of you the suspected scamming seller, so any option that puts them at much greater risk would be flatly refused unless they are idiots...
