Do you have any unpaid item strikes? This can prevent you from buying from many sellers.
Have you bought from this seller before and recently? There is a site preferences a seller may have inadvertently selected, to stop a buyer buying ** amount of items within a short period of time.
If neither of the above applies, send the seller a message explaining the issue you are having and ask them if they can put you on their buyer exemption list , that may help.
@adrian310384
