I am trying to put myself in the sellers place here and playing devil's advocate so please do not jump down my throat.
Why should the guy have the slightest idea that the clutch was about to fail? On your own admission it drove fine for eighty miles. The nature of an unexpected catastrophic failure is that it is unexpected and catastrophic. If he has even quite good mechanical skills I am struggling to understand how he could predict total clutch failure. Clutches, by their very nature, wear slowly over time and the normal driver is usually the last to recognise the symptoms whereas a new driver will immediately think "there's something wrong with this clutch" as soon as he tries it.
In my experience every car ever sold by a private seller has something wrong with it. Usually the owner has reached the point where it is the straw that broke the camel's back. You know the scenario, spent a lot of money, looking forward to a year's worth of carefree driving and then something, possibly minor, fails and tips you over the edge. "I am not spending any more money on that, ..... it has to go!"
Your job, as buyer from a private individual, is to try to identify the problems and make sure that you have some funds set aside to correct the inevitable issues that arise. Buying from trade gives you more assurance with a warranty, which you are paying for, of course.
I reckon the guy did not even know there was a problem with the clutch, or if he did he did not realise that it would lead to a catastrophic failure. Unless he has an ultrasonic tester and he has already looked for hairline fractures, of course.
If you had bought the car from me I would offer you your money back provided you delivered the car back to me. If I had no idea that the clutch was failing, and you had beaten me up on price, and IF I had got a piece of paper that said "sold as seen" then I would be sympathetic but I am afraid it would be caveat emptor. Otherwise known as "buyer beware".
I have bought some lemons in my time and it has always been my own fault. If you believed the clutch was heavy you should have walked away.