On the first day of the world, which lasts a full 24 hours, there are enough 'popular' aircraft for everyone to choose what they want. This first day receives several new batches of aircraft so by the time the second day starts and game days become 30/35 minutes, the vast majority of players if not all, will have an aircraft they want.
The second day is a bit of a rush as players can then place another order (as the max 2 per day rule prevents this in the first 24 hours). Often the best aircraft disappear quickly on this day so the ones with slightly inflated prices remain on the market.
Once things settle down and as in most new worlds, the market seems to run out of the best aircraft within 5-7 real days, hence the thread about the used aircraft market appearing in every world. They do appear, but demand is so high with airlines starting to make good money that they are snapped up by the players who can spend the most time online at these stages.
Prices aren't too bad, as the brokers only respond to the high demand after the initial surge, so for example a 15 year old A300 will be about 2.5 million and a 737-200Adv maybe 800k-1 million to lease. The vast majority of players will lease to start with as the only aircraft you can buy are little prop-jobs and it's pretty difficult to run and airline with aircraft with less than 20-30 seats, although some conditions do permit this, but that's another (long) story in a thread elsewhere.
The market will slowly improve as airlines turn towards the new aircraft market as profits rise and this allows for a more normal used market probably a couple of game years in.
Hope that sheds a bit of light on things
EDIT: I've now attached a screenshot of the Boeings currently available in MT#4 so you can gauge the orientation of some of the comments in other threads about the market conditions. As you can see, it's pretty minimal, but it's the same for everyone and it encourages players to opt for new aircraft. This helps slow growth in the world, as rapid and ridiculous growth was a feature of many early AWS worlds.