Someone would start to make or do something for fun. Someone else would see it and want one. A market is born. They try bartering for a while, that gets too complicated, then someone remembers money worked pretty well for exchanging goods and services.
Not necessarily. Lots of people already release their "fun" things they make for free. If you had all your needs taken care of, you could release anything you made for free a lot more easily.
That being said, there will always be limits even if you don't have to work, there are only so many major sports teams that can be owned by a single person. There can only be so many stays per night at fancy hotels in a particular city.
Even with biological needs satisfied human society creates social\cultural 'needs' that people will fight\work just as hard to acquire or maintain. 'Keeping up with the Jones' can create demand for literally anything. Unless we each get a genie that grants our every wish, we'll find something to compete over and markets will develop around those.
Historically, barter only tends to appear in communities used to a monetary economy when something temporarily disrupts the currency system. The more common pattern of commerce within communities not previously accustomed to currency is that goods are given in one direction at a time, and incur an informal social debt that may be eventually be repaid with reciprocal goods or by some other means entirely (e.g., support during community disputes).