I always thought that people dealt with car sales yards and trade ins to avoid all the hassles like these, the sales person or person doing the deal should have outlined these things before the sale..
To me it sounds like they have got upset they had to spend money on the car they sold you and want to try and recoup some of it, either that or they have put your old car through a WOF it has heaps of issues they didn't know about and want you guys to fix them..