Stars bought through the App Store or Google Play can be refunded by the store long after they have been spent. Apple allows up to 90 days.
Some sellers rely on that. They buy Stars, sell them at a heavy discount, then request a refund on the original purchase. The store gives their money back and Telegram removes the Stars from whoever is holding them. Telegram's terms allow this and no compensation is given, for the Stars or for the gifts bought with them.
It is worth knowing why a real shop can still be cheaper than buying in the app. The App Store price includes Apple and Google's 30 percent cut, so a business buying at scale has a genuine margin to discount from. A private account has no such margin, which is why an unusually large discount from one is a warning rather than a deal.
@StarninjaBot
Full breakdown of the scam - read the blog