When it comes to smartphones, I would almost always prefer to buy a new smartphone.
You may ask: why?
Well, first, new smartphone will get software updates for years.
Software updates improve everything, especially battery life and design.
New smartphone has new battery.
While I personally know how to maintain battery in smartphone so it stays good for years, most other people dont. Most other people will overcharge (above 80%) and let battery drain below 30%.
Used smartphones may malfunction.
When it comes to used smartphones, you will mostly get them online because the greatest amount of offers is online. There are also stores which sell used smartphones.
But with this, there comes a problem of possible malfunction. In fact, you might not even notice that smartphone has some malfunction until few months after.
Used smartphones are outdated.
Everyone knows that new smartphones use latest technology. While used smartphones may be cheaper, they are basically an older technology. This affects everything from battery life to performance.
So I would almost always prefer to buy new rather than used smartphone.
Does same apply to all other products? Well, in some yes, in some no.
Cars you mostly want new, for pretty much similar reasons as smartphones.
Any transport device, you want new.
Computers are kinda better used, unless you are after some latest technology for gaming.
TV is in most cases better used, unless you are after some high resolution.