Credit Reporting -Accounts Sold & Reported after

I have a serious credit reporting question which I hope someone can answer.
Ok, obviously, companies charge off bad accounts and later sell them to a collection agency or company. Once the new owner of the original account begins to attempt collection and fail this second owner will find yet another buyer. Are you with me so far?
My question is this: How do credit reporting agencies classify these resold accounts? Are they counted as a new debt by the new purchaser?
In this respect, if they begin to appear as a new debt, then these accounts can plaque a person's credit history and report for a much longer period than a mere 7 years. This seems to be rather unfair reporting, or is it?

That is some interesting paper trail work there. I have been having issues because of something from like a hundred years ago has popped up in my mailbox. One)It is not MY name nor a name I used in the past, Two) I do not trust the validity of the debt they are asking me to pay.

I am at a loss here as what I should do next. I don't know who the company is, it appears to be a collection agency, and I don't even know who they are claiming that I owe money to. Maybe it is a fraud, but even so, how did they get the privilege to see me apply for credit recently and to see my details. I am starting not to trust some of these freebies I've been getting.