Most sell it on because they know its dodgy. No paperwork, or they know theyll never be able to collect. So they double dip when selling. They write it off against tax, and get money from the buyer.
DCA's do the same. They just want money, If they cant get money off the debt within their own timeframes, they sell it on to some other DCA to try.
Sadly not many people know their rights in regards to debts, So DCA's buy debts in their tens of thousands, of not more. If only 10% get tricked into paying, then its a bumper pay day for the DCA and they have a massive profit.
But theyre greedy as hell, so lie, threaten, intimidate and misuse the legal system to get as much as possible. Remember, they buy debts for maybe 10% of their worth, then try and chase for the full amount, giving them a MASSIVE return on each one paid.