Link removed. As much as you're (@Aj3537) answering a question, it just sounds like the seller isn't being truthful, and as such,
no need to link back to said seller.
PKHeX is presently a PC program (there are mobile versions, but let's not touch that for now), and you can't install PKHeX on the 3DS.
There are tools that can back up saves from a 3DS (to be used on PKHeX on PC),
and tools that help you check stats, or import between generations (PKSM).
Maybe the user simplified the description for noobies, saying nonsense like "PKHeX on 3DS",
but I repeat: PKHeX cannot be installed on the 3DS.
A few things can happen: spin-off fakes, imitation in usability or similar usability, or misrepresentation.
edit: To add on to this, a while back someone tried to make a port to 3DS, but it's called PCHex.
No idea how the development went, though.
Also, you're asking the site that has the dev for PKHeX on.
If the dev were to make it available on the 3DS, we would know.
As such, until the dev comes out to say that there's a secret project or less well known project, believe me when I say there's no 3DS version of PKHeX from us.
Go to their various pages and grab the Homebrew files, and load them up in homebrew launcher. Once your 3DS is hacked, getting these are rather simple.
edit again: No need to keep telling the person to go to that site. It sounds like unwarranted advertising at this point.
Also installing CFW and homebrew on your own 3DS is a piece of cake.