Of course we are not talking everything on the car, but selected car parts that have been upgraded or sourced from a cheaper supplier on a continuous basis to either improve the car or build it cheaper. The results are that every part now needs a serial number or unique reference part number, which sometimes is very hard to locate. Often the best way to match up your car part id to take the part off and take it to the dealership and so match it with a new one. Unfortunately you cannot do that if you choose to buy your car part online.
One solution is to join one of the many online forums that dedicate members to the particular make and model of car you own. Then ask the question first before you buy anywhere. Is the part I need to buy have different serial numbers or has it been supplied on a consistent basis around the time of manufacture could be a question?
It may be worth the hassle, as many people now choose to buy car parts from the many locator services, which put you in touch with national breaker yards. You get the buy a used car part cheaper, but the part is likely to be made to the manufacturer specification and not a cheap copy that you often get through accessory shops. The problem here is these used car parts do not come in a box with part numbers written on, that can be matched against the manufacturer manual.
Some manufacturers are worse that others, some may bring our several versions of the same car each year to help with its marketing program. Luckily others only change parts with full research and good reason to do so, of course there is the argument that they may get left behind with new technology.
At the end of the day, if you buy a new part from a shop you could argue you were sold the wrong part should be wrong. But if you buy online, it is likely that the responsibility will lay with you, not to mention postage costs. So take your time and do some research first.