Just because nobody has bothered to actually take a class action to trial and succeed on this does not mean that you are right. The example you give, A$$et, is a nasty litigator, who will stoop to vicious tricks and legally questionable moves that could stop even a class-action attorney or law firm in their tracks!
Have you ever researched into how factors work? They never buy accounts sight unseen; if they buy on a regular basis from a company, that happens only after the company has proven that their receivables are good. Another thing: There would be a full, enduring recourse provision in any competent factorer's contract.
If a "bad" account gets by the pre-screening, they can return it to the originator. And, do; the ones a factor sues for are those that go bad after they have completed the sale and have actually serviced the accounts themselves.
JDBs almost never have that form of relief--recourse-- available to them. When they do have it in the sale contract, it is a limited recourse provision that is normally only available the first time a portfolio is sold, and even that is very time-limited, ending as little as 30 days after the sale. Neither are their data streams usually "vetted" ("cleansed") to isolate out the accounts that are still likely to be collected upon; there would be a lot fewer uncollectible accunts being chased down if that were the case.
There is a good reason the "good" defaulted, charged-off accounts don't end up in debt buyers' hands all that often: Those accounts are removed by the OC, or the OC sues on them themselves. Even a first purchaser JDB or debt broker gets the accounts that the OC has determined are not worth holding onto or suing upon. In other words, they get trash, garbage, and just plain junk. Not that one cannot "pick" through the "garbage" and get something out of it, any garbage hauler--or curb picker--knows that, and their JDB counterparts do, too. However, ones such as A$$et would not be one of those JDBs, since the price they would have to pay for the portfolio is higher than they are willing to pay to get it. A$$et and their ilk are notorious for buying stuff cheap, which would be after at least one JDB has already "skimmed" the "cream" accounts. And, the proof is no longer accessible or available at any price.