While your going to buy a second hand mobile first you need to know that why sellar going to sell it because it is damaged or he is selling it fof money.
If he selling for money then check mobile for scraches and software problems .
Check present price of the mobile on online stores or in nearest cell point.
Check the speaker for sound quality.
Go to expert and take suggestions like I can buy or not buy. If it can buyable then ask how much you can spent on the item.
Try to buy for low price .
Try to buy latest os mobile