If you are willing to bye a good mobile under very loe cost then you should go for second hand mobile.
You will need to check some properties to bye a second hand phone. These are : -
Check for the original price of the mobile at the market. The seller may demand extra price.
Check how older the phone is and then offer a suitable price.
Ask the seller whether he is giving the mobile accessories or not, if no then offer less price.
Check the all parts of the mobile, if any thing broken in the parts or not. Check the screen properly if there is any ki d of scratches or not.
Check the Ram size, Internal memory, check the processor and most important check the bettery. The bettery should be in good condition. Like it can fully charge and give the suitable performance.
Check the loud speaker and ear speaker and check the microphone also.
And finally dpe di g all of your diagnosis offer a suitable price.
Best of luck.