In our area, the agents recommend that the buyer has a septic inspection prior to closing to help prevent this problem plus we have a special addendum to our sellers disclosure that is filled out for homes on septic systems, homes on wells, etc.
The current owners may never have had an issue, they may have been unaware that there were problems with the system. The contractor who came out, might be mistaken about who he talked to before or the owners you bought from, might not be the same owners he talked to. Bottom line, typically the buyer has to PROVE, that the seller knew and purposely did not tell the buyer. You have to bear the expense of fixing the problem and taking them to court. NOW, if your home owners insurance and/or your home warranty cover the issues, USE THEM. Then let them discuss the issues with the sellers insurance company or seller.