No seller here ever gives OC warranty to a buyer. You will have to understand that. They will tell you how much a CPU/Mobo/RAM/GPU can do, but no one will give guarantee for it. Unless the seller specifically states that he will honor a guarantee for a specific overclock, a buyer will loose any seller warranty he has, once he over clocks.
You tested your card at the default settings and you found it to be working perfectly.((no no no man, i just installed the card and i message him that its working great thats all, it gave the same thermal problem from the beginning, after i applied the AS5 i asked him about the overclocking)) Once you overclocked the card you lost any testing warranty the seller has given you. It does not matter even if the seller was using it at a even higher overclock.
As for non-stability of an overclock, it depends on several conditions under which he and you are using it. For example if one person does 380 Mhz, in a cold place/ winter it does not mean another can do the same in a hot place/ summer. There are a lot many factors which influence oc stability.