who can actually sue

CA's cannot sue since collectors are not attornies. Neither are OC's. CA's, OC's and secondary buyers can refer the debt to an attornies office to file on their behalf.

I think they were asking who has the right to sue. Basically a CA who is collecting for another does not have the authority to have a lawyer sue unless the owner of the debt has them do this. The only CA or JDB who can hire a lawyer and sue are those who own the debt.

Only the legal owner of the debt can initiate a lawsuit. Most CA's are simply hired to collect (ie they don't own the account), and so it's not up to them to sue. But if a CA bought/owns the debt, they could file a suit.

Some CA's do have in-house attorneys and can litigate/file suits on behalf of themselves or their clients.

My company has used CA's before that ultimately filed suit for us. They work the account in collections for a while, make phone calls, etc. If nothing happened with the account while in collections, they'd recommend filing suit and ask us for the go-ahead. If we agreed, then they'd go into court and file a suit.