In order to be able to buy from these vendors you must become a member of the Tenshodo.
They only buy a select few items from players, and they pay more than regular vendors.
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.
Merchant List (hours)
- Bastok (1:00-23:00)
- Jeuno (always open)
- Nashmau (1:00-23:00)
- Norg (9:00-23:00)