In order to be able to buy from these vendors you must become a member of the Tenshodo.

These vendors mainly sell items that are associated with Ninjas, Samurais, and Monks, as well as some other items.

Prices are based on your Norg reputation. The higher your reputation there, the lower your prices.

They only buy a select few items from players.

Tenshodo Merchants aren't open all the time either (much the same as Guild Merchants). They are only open during certain hours of the day. Each merchant has different hours.