The best way to make sure that you are buying a good santour is to bring a specialist. Someone who can distinguish between a good and bad santoor. Someone who is good at tuning and réglage.
Here are some buying tipps:
1) The strings should be new and not dark, otherwise they need to be changed.
2) Check the saddles. They should work properly and keep the strings tuned, otherwise the instrument needs réglage. So you should tune the instrument and check all the saddles.
3) The bridges shouldn’t be too short or tall. All of them have to have be the same, if not, the instrument is not balanced, and you should Exchange some or all of them.
4) Check the appearance of the instrument for any crack or extra line.
5) Sometimes the instrument has some hidden crack and it's not recognizable as long as the strings are fixed. You can't know if there is any till you detach all of the strings.
6) Examine the plate for any kind of wood swell or shrink.
7) Was it tuned before or it was untuned all the time. A santour which tunes has a better sound than a one that is not tuned at all.
8) Does it make a wood scrap sound? Is the clarity of sound good or bad?
9) After checking all the mentioned details, ask for 48 hour test time. In this period tune it several times and play with it to make sure there is nothing wrong with it. If possible, show it to your teacher or a specialist.
Obviously a second-hand costs much less than a new one. It is good for a person who can't afford a new one. Some of them are not worth buying. A new Santour has new strings and intact saddles, it also tunes easily and has less trouble in total.
Be aware that not every second-hand santours are bad and with a little help from a specialist and doing some researches you may find a good one.
Here you see a second-hand santour with several problems:
And here you see a second-hand santour with good appearance: