Perhaps they are restricted because of your established buyers requirements. Most likely because they have 2 or more Unpaid item strikes in the past year or location.
Under Buyer Requirements there is a setting where a seller can limit the number of items a repeat buyer can purchase in a set amount of days. By any chance have you enable that setting? See the policy link below for more information on Buyer Requirements. (This link is also provided in Message #3 fyi)
