Responsible if your debt is sold?

by Rory
(Colorado springs,Co. Usa)

I was wondering if your debt is sold to another collection agency are you still responsible to pay the debt.There is a company called Chad Steur Law(claiming to be lawyers)but in fact after a little research on my part found out that they are just a collection agency fraudulently claiming to be a law firm.who has bought an old debt of mine.After my research it seems that i am only responsible to the original creditor Not the company claiming to be a law firm whom has bought my original debt. Not sure if they are breaking the law or not in claiming to be a law firm on order to scare me into paying this old debt.So mY question is, am I responsible to pay this company who was bought my old debt?