And for that before you think about buying used products you should take a look on these steps to not waste your money.
1- Buy from trusted peoples
One of the fault that many peoples do is they buy anything from anyone , so if you want an advice from me : don't buy from not trusted peoples .
2- Know everything about the product
For example if you will buy a new computer (i.e: macbooc pro 13inch) you should know it performances and features because this used computer can be modified with less performance drivers (processor,GPU...). And the same thing for other products .
Another thing , always ask about the product before you buy it -- ask about performances , how many years it's used , it's all original or not , with all accessories or not , what are the negative points in the product....
3- Try the product
Before you buy any used products "TRY IT" don't buy like that , because after you pay money you don't have any chance to change it . So ask the seller for 1 or 2 days to try the product before you buy.
4- Give it to a professional
So after you get the product to try it for some days , i advice you to give it to profesional person to take a look on it and see if it's good or bad product .
5-The price
Sometimes with the price you can know that it's a bad product for example you will never find a macbook with 200$ (it's just an example) .
6- Ask about return money or garantee
If it's possible , ask if you can get your money if the product don't work anymore before one month of the buying date , this can be a good choice if you want to save your money.