This obviously depends on a family to family basis, but from my experience it depends on the history you have with your parents, your education as well as your age. As an example, my grand parents really only started listening to some of the stuff I have to say about certain things after a few years in University (they're old school Korean), even still they will take my Uncle's and Aunts' advice with more weight probably due to more experience as well as they're multiple degrees in various fields.
Since your parents have known you since birth (and creeping your OCN profile showing that you're 13), don't take this the wrong way, but to them you're still they're little kid who used to run around the house in diapers. I should mention that this is different then let us say, your friends parents who may only know you as that kid who is "good with computers" and that may be their only experience with you and thus your advice to them may weigh more than some random salesmen.
In most cases parents think they know better than kids, right or wrong, you're still their little guy/girl and they feel that they know you better than you think and proving them otherwise in matters like this at your age can sometimes be an uphill battle. Basically what I am getting at is that they are "appealing to the person", they are rejecting what you have to say because you are their child and they think they know better.
What is most important in any argument is to always leave a way for someone to save face (similar to Sun Tzu leaving an escape route), in an argument even if someone is wrong and they know it, if you have them backed into a "corner" they will fight with all they have to save face. Calling them stupid for not believing otherwise will not help your situation. Your parents seem to be doing what most people do, falling into the fallacy of "appeal to the expert" with that salesperson, similar to how in commercials they have for medicine they have the actor dress up in a white lab coat to make them appeal to be smart and authoritative in the matter.
Right now there is nothing you can probably do about their laptop purchase, if you try to prove them wrong it won't do anything since they have already decided to purchase the laptop and will be subject to embarrassment either from a "I told you so" whether you say it or not as well as the simple act of returning it (since returning items is generally a pain). In all honesty for future note you should try to level with them and explain things to them in a way they can understand, if you have to also refer them to a source of material that they can look at themselves that supports your claim.