There are a lot of things to check during a new bass purchase. I want to share some of the most significant things that I look for.
Each bass comes from a different set of trees, and have different wood / resonance / sound qualities. Basses from the same brand & model may wildly differ from each other. Therefore, it is the buyers responsibility to detect uncorrectable issues.
The points below assume that you are checking a bass with a good setup and fresh strings.
Trauma. First of all, check the bass for any significant dents / cracks. Wear on the finish (relicing) is acceptable if you like that kind of thing; however, trauma on the wood means that the bass has been dropped or hit somewhere. It might be risky to get such an instrument.
Weight. You will carry the instrument all the time; in your gigbag or on the stage. If the bass is heavy for you, it will leave you exhausted after a 4-hour show; and possibly damage your body on the long run. Ensure that the bass feels light to you.
Neck profile. Depending on your physical attributes; some necks will feel good in your hand and make you play effortlessly, while others will feel bad and make you struggle playing. If you don’t like the neck, skip that bass.
Truss rod. Starting from the ideal neck relief state, the truss rod should be adjustable to both sides. If the truss rod is stuck and won’t turn to either side, this might be a very hard to fix problem. I would skip such a bass.
Dead strings. Some strings may not sound as full as the others. Typical cases are weak G strings or dull E strings. Note that such issues might be resolved by a better setup or mindful string choice, but if the issue is caused by wood density, there is not much you can do about it. Such basses are better left at the store.
Dead notes. It is typical for many basses to have notes which lose their sustain rather quickly. That’s caused by the wood density cancelling out some frequencies. Typical problematic area for dead notes are between the 4. – 9. frets of the G string. Most basses will have some dead notes. If the situation is not very dramatic, the bass can be acceptable. However; if the note dies out very fast, the bass may not be playable at all. Some products (such as Fender Fat Finger) promise to resolve the problem by adding to the mass of the bass, but I recommend skipping basses with dramatic dead notes.
Wolf tones. Similar to dead notes; wolf tones are notes which come out very loudly – not unlike a microphone feedback. That’s caused by the wood density resonating & empowering some frequencies. Similar to dead notes; I would skip an instrument with dramatic wolf tones.
Note choices. The sound & feel of each bass will affect your note choices and make you play a little differently. Pick the instrument which makes you play better.
Correctable issues; such as fret height, nut depth, electronics, etc can also be checked obviously, but they wouldn’t stop me from purchasing a rare great bass.