This occurs if the metal inlays that make up the frets get worn out or sink into the wood. Compare with other guitars in that shop. It should be absolutely straight (look from the sides).A fretboard usually curves in front over time due to the constant tension of the strings. This is especially true for 12 string guitars.
There should not be such that the metal dividing pieces are set too deep - if it takes too much strength to hold on to a bar chord, the fretboard is probably old or badly made.
The wood should not feel too light as any light wood here will bend in a few years.
Make sure that everything is smooth and there aren't any jagged projections. This is obviously more important for the safety of your hands.
The "truss rod" is a metal strip that runs along the inside of the guitar's neck. It is used to compensate for the tension of the strings that change to the curvature of the neck over time.
The junction of the head and the neck has a metal structure called the nut through which the strings are threaded. Check if it is alright. You may miss a crack on the plastic material there. Look particularly at the edges. These edges are the first ones to break off and cause a hell of a trouble for both playing and tuning the guitar.
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