An easy tip to buy used or non warranty phones
Watch teardown/disassembling videos of your required phones, and while purchasing, pay special attention to those areas from where the set is opened/disassembled for repairs and LCD replacement. Checking screws only do not tell everything, as they can be replaced with new ones, or maybe hidden in non removable battery phones.
P.s, i was selling a friend's HTC M8 and one person acting over-smart kept checking non removable back cover fitting (not phone labels on back cover) repeatedly to see if the set is sealed or not. But in reality, M8 is opened/dissembled from the front side after removing stereo boom-sound speakers grills, and he did not check that specific area at all.
There are also certain models, that stay unsealed (untouched screws) even after LCD replacement.
So people should do their homework before buying used phones :).