I hope this is single isolated incident, typical chinese manufacturer have ridiculous warranty, compared to normal western companies.
Usually sellers wash their hands unless it is dead on arrival.
Where I live, even if product has its warranty expired, then one can still claim it to be fixed/replaced by manufacturer/importer because every product has its
reasonable expected life cycle. When a washing machine (for example) has 1-3 years warranty, people still expect it to last for at least 5 years or more, when their old good quality washing machine with all electromechanical (not much if any electronics) lasted for a good 10-20 years.
PS3: I can fix it with messing around the trim pot like http://gushh.net/blog/ut61e-calibration , but the same problem happen after a day or two.
Does your multimeter get banged around a lot? From what I have read, the pot is really sensitive when adjusted. I think some people have replaced their cheap chinese copy trimmers with genuine good quality ones, have you considered that? Do you see anything wrong with the multimeter pcb? Does it have flux residue or something on it?