Payment & no communication

Ive sold my iphone 13 pro to phonetradr 3 weeks ago they had advised that payment will take 5 to 7 working days , they gladly collected the phone but since have not paid. Endless emails have been sent with no feedback , after 4 days an email was sent it will take 7 to 14 working days . After the waiting period still no payment . I have sent another email they responded with system change over as an excuse . I still have no payment to date ! . These people are unprofessional , ********* & absolute ****S &, ****MERS . I will never purchase or sell to them again !! I advised that they should return my phone or give me the money owed to me