From the Users: Users page, find the User/bidder and then click on the 3dot Action menu in the far right corner and select, 'Enable Cash Buyer'. Or, click onto the User number to view their profile and from the right column select the toggle switch called, 'Cash Buyer'.

If this is selected, on the Invoices tab this bidder's invoices will have the dollar symbol ($) shown under the 3rd column called Bidder Details. This can be handy for clients to instantly know which bidders normally pay via cash so you don't charge their credit card on file.