I also not seeing if OP is meaning just using some random web browser to buy items on random web stores, or talking about selling a computer and question what OS should be installed when sold to some stranger.
If first option, then the answer is already given. But I also jut want to add that safety within web browsers and password management I think are more important than the sheer choice of OS. Of course, with Windows it's easier to just snag a random exe files from the web that is being described as being able to do amazing stuff - compared to Linux. But in the end, the problem isn't about the OS but rather what is in front of the computer screen leaning over the keyboard. For both OS, it is
possible to deny any updates for it, and it probably doesn't matter - it is unsafe anyway regardless of the OS.
If the second option, there is one prerequisitement needed in order to answer:
- If laptop or bought with OS preinstaled. Here is two options.
- You can either restore Windows back (not remembering the precise way, but easily to google for it) to it's original state. I did this with a laptop at work where two piece of software was acting up against each other and caused problems (no, it was not any kind of anti-virus as you might expect) - so I did this a couple of times in order to see what combinations of installed programs that was acting up on each other.
- If prepared when brand new, use some software to create a disk image. The disk image must be located on another unit, normally a usb hdd, and use same software to restore the contents before you sell the computer.
- Otherwise, you can probably just install Windows without a license (let the buyer pay MS) or you can buy it yourself <disclaimer case see comment below>. Installation of Linux is of course also possible, but probably less potential buyers.
disclaimer: As I have use only Linux at home for a while (the last Windows version I bought was W7) so therefore I haven't investigated in the options (cons and pros) of installing a licensed version of W10 with the purpose of selling it.