vehicle parts

Hi all,

having been an Ebay member for 23yrs starting to get somewhat confused, so you need a vehicle part and you go in to my garage with a vehicle details are then type in what you need and up comes this fits your vehicle so you purchase it and upon arrival no way will that part fit twice this has happened to me, now i am searching for another item and Ebay says this is a assured fit? then reading further down the page it states if it does not fit it will be a free return. how on earth can it be an assured fit in the first case i'm getting very reluctant to purchace anything for my vehicle's  in future.

my cv ex motor vehicle tectnician 55+ years in the motortrade now reti