As a buyer, you want this security so that you can pay a few thousand $ without the fear of getting nothing in return. As a seller, you have to treat the 21-day float period as a cost of doing business, or else impose less generous terms of your own (and see less interest from wary buyers who do not want risk). In the business world suppliers are often expected to provide 30 or even 60 days of credit to commercial customers, and dealing with this is the essence of cash flow management.
It seems you want all the benefits of having your money instantly, but that someone else bears all the risk. The opportunity cost of the 21-day wait is the risk premium you pay for access to a large marketplace.