Cashapp I sold them 3 gift cards for $900 and only got paid $150

 I sold cardcash.com 3 gift cards for 900 and they took almost 2 weeks to give me a solution which was not to my liking.  They claim that I had previously sold them some gift cards 3 years ago that went bad, and that I owed them $750.  So i would only be receiving $150. 

I countered that i had an established account with them, credit card on file, they had my contact info, and they never contacted me.  I could have disputed it at the time of incident.  My gift cards were brand new, never compromised.  They told me there was nothing they could do for me.  So I will never do business with them again.