Pabili ng ube = Please buy ube/purple yam.
It's a polite request already because the imperative command would be "Bumili/Bili ka ng ube" (Buy ube) or "Bilhan mo ako ng ube" (Buy me ube).
But in most occasions, people use "pabili" in place of "pagbilhan" (sell someone something) which you'll often hear in stores. So if you say "Pabili ng ube" to a storekeeper, he'd take it as "Sell me ube/purple yam."
Good luck! :)