I came across this meme on a page a while ago. My reply to the author:
Unless you want your readers to buy their first guitars online, don't say unless you don't want.
If you want the readers to avoid buying their first guitars online, just say unless you want. Or say, If you don't want . . .
Or else, simply say,
"Don't buy your first guitar online, or else you will waste your money and time."
