The daily counts show everyone the relative progress or strength based on a number of categories. Wealthiest nation is one of them. It tells everyone who has the most money and the relative levels such that you could calculate everyone else's cash reserves.
Whenever you make a trade offer in the market you lose access to whatever resource you are offering in trade. If you make a trade offer with cash you 'lose' that cash to the stock market. In the event someone accepts your offer you get the resource you wanted to trade your cash for.
At any time you can cancel a trade offer in the market and get your resource back
It is my belief that the wealthiest nation count only counts cash on hand, not any cash offers you have in the stock market. Those funds are tied up. Making a low offer (1 cash for 1 food) ensures no loss of cash.
Why would you do this? To make others believe you have no money.
Why would you want others to think you are broke? Maybe so you can get a favorable offer on a private deal. "I don't have a lot of cash so I'll just give you goods and my good will."
Maybe you want people to underestimate you. Or you want them to tread carefully in their messages to other powers because you may have spies on neutral nations. (this might discourage someone from looking for a coalition against you to betray you)
Note: it is not always good for others to think you have no money! Them underestimating you may cause an invasion you wouldn't normally have to deal with. Maybe spending all your money is a sign that you are active and will be a threat later. There are as many possibilities as there are players.
I think this trick is a good tool to have for when you think it will work for you. It is not my default action, however.
Cheers
