Outlaw,
(1) Theoretically, yes, they can just keep selling it to anyone who will buy it. I think if the buyer knew they account was disputed and unverifiable, they wouldn't want to buy a "bad" bad debt. But whoever is selling it probably does not disclose that kind of stuff to a potential buyer.
(2) It is the "owner" of that debt that can charge interest. Generally, if a collection agency is actually collecting for someone, it is not the collection agency that calculates the interest.
In a case where a collection agency buys the debt (aka junk debt buyer), they are obligated to charge interest according to the law. But I don't think that they really do.
Suppose you have a credit card with Chase Bank for 18% apr. Chase Bank is a licensed financial institution, and can charge you whatever interst they want. Account defaults, and Chase sells it to XYZ Collectors, Inc. XYZ takes the account and says, "Chase was able to charge 18%, so we can too." BUT NO... XYZ is not licensed to charge interest (just like the internet PDLs). So for them to charge interest (above the statutory rate) is illegal.
Somewhere you need to lookup your state laws to determine what the statutory rate is.
This is what my problem is... Chase charged off my account back in 2004, and stopped calculating interest (they showed the same balance for over 2 years). Resurgence bought my account, went back in time, and re-calculated interest from the date Chase charged it off, at the rate that Chase was able to charge. So I disputed it, requested validation, and they decided to sue me. I guess they are hoping I don't go to court so they can get a default judgment. I think they will have a nice suprise when they found out I filed a counter-complaint.