Is this legal? Can a collection agency who bought the debt from the origninal creditor sell the debt to a new?

collection agency while I am requesting that the debt be validated? I sent a Debt Validation request to a "collection law firm" representing a collection agency (which bought the debt from the original creditor). I sent 3 request (certified return receipt) and finally recieved a response after I the 3rd which stated "they were in violation of the FDCPA, and others, because they hadn't responded to the validation request and were still sending collection letters and the collection agency which they represented was still reporting on all 3 of my credit reports. When they finally responded with 2 statements (which did not indicate my payments to the company) and a document that I could not read which i guess is to be the contract with the Original creditor (credit card) did not have my signature, I noticed that collection law firm now indicated a new company which they are representing for the debt. I had not received any other notification in regards to the new company besides in the response from the collection law firm showing "Re: the new collection agency name" and just 18 days ago I had received an attempt to collect from the collection law firm (which I noted above) which stated "Re: the other credit agency". So far the debt has been sold 3 times since June 2010. I was in dispute with the original creditor (credit card company) when they reported and sold the account to the collection agency. The original creditor and the collection agency (who purchased the debt) were both reporting on my credit reports until I disputed the original creditor last month with the 3 reporting agencies and it was just deleted from my report.