He refers to the hugely up to date London Market Traders Act of 1667, sections of which have never been repealed. I cannot verify the actual date of this Act as Google ain't got it. But apparently, enshrined in it is a reference to 'paying on the nail'.
The custom, according to my bookcase research, dates back possibly to medieval times when shallow vessels were mounted on a stand and business was concluded by payment into the vessel. (Predating cash registers, obviously.) It is thought that this stand and vessel may have roughly resembled the shape of a nail.
"If, for example, I said I had a turnip, a gold bracelet or whatever,"
says Glyn, "and I asked what am
I bid and someone placed money
on the nail, which I decided to accept, then this would be known as 'paying on the nail'."
Apparently, such 'nails' or pillars can still be seen outside the Corn Exchange in Bristol (unless they have refurbished the Exchange since my 1998 edition of Brewer's Dictionary of Phrase & Fable). Perhaps some Bristolian might be good enough to enlighten us further.
So, if your epos were to go down, you could always give 'payment on the nail' a go. It might play havoc with your tax returns, but on the other hand it would almost certainly get you some free publicity in the form of a write-up in the local paper.