It's a little tough with the smaller guitars as they are aimed at kids hence they are cheap.
You need to take a guitarist with you, go to some guitar shops and see what the seller says and what the guitarist says.
A guitar is made of wood, wood gets more expensive the better it is, better guitars use better wood which doesn't twist like cheap wood, most cheap guitars have twisted necks and a lack of adjustment to get the neck straight to bring the action down to where someone can comfortably play it.
The difference is night and day, hence buy second hand, the wood has had time to move about and settle, new guitars are more unpredictable, they can start out nice then turn shit.
A wooden door is a good example, buy a cheap door from wickes and it'll be all over the place, contracting and expanding with humidity and coming apart at the seams
There is a lot of tension on the neck once all six strings are tuned.
Cheap guitars are basically pointless.
