The Garden takes a huge chunk on merchandise sales, so I suspect it is not cost effective to design and produce event-specific merchandise, ship it in, and sell it. Even at a large mark-up, I imagine it is break even at best with a risk of loss due to unsold shirts, etc. The only way to make money in the Garden is to sell "tour" shirts where your low margins are offset by high margins in other venues (and your unsolds can be shipped to the next town).