I have bought and repaired many vintage turntables over the years. I can count on one hand the number of turntables that didn't need quite a bit of time and effort to get them going properly again. So, when you are looking to buy from somebody, you should ask a few questions:
1. Does it spin, and are the speeds accurate and stable?
Very important question because if it doesn't spin, or doesn't spin at the right speeds consistently then it's not a good start. It could be as simp[e as replacing the belt (for a belt drive turntable) or it could be a complete turkey.
2. What condition is the stylus in?
Unless someone is selling the turntable and states the stylus is brand new, then you should factor in the cost of buying a new stylus or replacing the cartridge completely.
3. Does the platter wobble?
Sometimes the platter or spindle gets bent and when your record spins around it will visibly go up and down and create sound distortion. Best to avoid any bent stuff
4. The auto- functions
Some turntables are so complicated they can do 100's of things and many were gimmicks at the time and depend on lots of different scenarios for them all to work perfectly. However, the most common automatic function is auto-return. And you should be asking if the tonearm returns at the end of the record (after the music has stopped), if it returns back to the tonearm rest and of course does it lift the tonearm up properly and not drag the stylus when doing do.
There are many more questions to ask - but more later