The buys are given out depending on your placement at previous Nationals competitions, or in the case of some of the dance and partners, they are given out due to lack of competitors. In order for most skaters to qualify for nationals, you first must skate at your regional competition. At the intermediate and junvenile levels, these skaters then move right onto the Junior Nationals. At the higher levels, these skaters must then compete at sectional competitions and the top 4 skaters in each event move onto Nationals. I believe most of the top level senior skaters did have buys this time around. The only one that I'm aware of who had to compete at regionals and sectionals was Johnny Weir.
The Grand Prix is made up of many international competitions. Your placement earns you points and those skaters with the most points are invited to skate in the Grand Prix final.
Tammi