Your reseller is trying to get out of replacing the CPU and doing its responsibilities.
Overheating doesn't cause bent pins on a CPU. As far as I know only physical force on CPU pin can bend it. Like, for instance, if you rip out the CPU from the socket or you drop the CPU on a hard surface or you forced the CPU in the motherboard's socket in the wrong direction while installing it.
I bet since the Reseller mentioned the Thermal paste was dried that when the Reseller tried to remove the CPU Cooler he ripped the CPU out of the motherboard's socket bending one of the pins. It is possible the CPU Cooler became glued to the CPU thus when he tried to remove the CPU Cooler he ripped out the CPU from the motherboard damaging it. Just my guess.
If the Reseller said the CPU has bent pin(s) then it was the Reseller who damaged your CPU and should replace it themselves. Since they were the ones who installed it in the first place and you didn't touch the CPU in anyway like removing it or removing the CPU Cooler etc.
The Thermal paste could have gone bad but that is not your fault but the Reseller who assembled your PC. Maybe he used inferior Thermal paste or applied it incorrectly.
The Reseller is telling the truth about the AMD 3 year Warranty being voided if your CPU has bent or broken pins. But there is a excellent chance with the correct procedure and tool to straighten out the bent pin without breaking it. Broken pins there is not much you can do except hope it is a pin that doesn't affect the CPU performance or operation.
The Reseller is the one who built your PC thus the Reseller is responsible if any parts that are defective or installed incorrectly. If you didn't open the PC computer case and messed with the parts inside you are not responsible but the Reseller.
It is possible the another part became defective like the Motherboard or PSU etc.
Did the Reseller give you a Warranty on your PC built by them?
EDIT: If you reside in the European Union you have certain rights and a 2 year Warranty from the Reseller.
But If not from the European Union, do you mind mentioning which country you are from?