I'm having this problem too. Came here to see if anybody else was frustrated... glad to see you are!

And, just to be clear about everything... this does not appear to be a problem with availability or anything. It seems to be just a website bug -- the "Purchase" button has disappeared. I'm trying to waitlist an instrument, and everything works fine except there is no "Continue" or "Purchase" button. Like they made a car but forgot to add a place to stick the key in and start it! (Yes, I know all new cars are pushbutton these days... bear with me... grump, grump, kids these days...)