How Can We Make Sure U r Buying orignal Product

hi Alls....

Well now a days there is alot of used ithem as well avaible in karachi's market and people of karachi preffering them on the new products made by chine due to this because the used one's are branded well i need a laptop in any condition but i want to know who can i make sure that the product am i pruchasing is orignal and also need some knowladge about laptops confiriguration.....