Actually potential buyers don't want to wait until a set number, e.g. 3, sales are booked before the tshirt is even printed and then later shipped. No body will want to give away their CC/DB details, and then wait for 7-30 days before the shirt is even printed and packaged, and then additional 4-10 days to receive their purchased t-shirt in mail. Most of those FB users who are jumping in enthusiasm initially ("take my money...", "I wanted it like yesterday!!!...") are not buying may be because of the above reason/s.