28 days is because this industry used to work on trust and a gentleman’s agreement,before all the flyby nights who have no Conscience got involved.Money up front a month before the combine if sold privately
or put it in auction with reserve .It will be the best 5% commission you ever
pay an auctioneer .This is pretty basic stuff to be honest and cant understand the
logic of payment 28 days after cutting,that is very risky business.