The internet makes things much easier nowadays with instant quotes and the ability to buy anything including car parts online 365 days a year 24 hours a day. With many products it is quiet easy as there may be just ion version of what you are buying, but when it comes to car parts it can be a totally different story. There is this assumption that by buying a particular make and model of car, it will have the same set of parts that that entire model would have. The truth is this is juts not true. First of all the time when your car is registered, that normally dictates the year of the car, may not used auto parts be the time it was manufactured. It is very possible that your car has been standing for 6 months on an airfield somewhere, before it was eventually sold and registered at new. There can be the same car, registered at the same time but was built 4 months later. This ultimately means that there could be a difference in the parts that make up the car.
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.