Nha Trang is not a dangerous place and most people selling things here are straight with you. But like any tourist city it has a handful of recurring ways to part visitors from their money, and they are worth knowing in advance.
Seafood by the kilo. This is the big one. The lobster you point at may weigh more than you assumed, and the posted price is per kilo rather than per animal. The fix is simple: ask for it to be weighed in front of you and confirm the total before it goes to the kitchen. A decent restaurant will do this without blinking.
Suspiciously cheap island tours. A low headline price is usually recovered through a crowded boat, short time in the water, and extras: island admission, snorkel gear, a sun lounger. Ask what the price includes before booking.
Taxis that don’t run the meter. Use a ride-hailing app so you see the fare first, or ask for the meter as you get in.
Freelance “guides” around Po Nagar and the Long Son steps — someone attaches themselves, walks you a few paces and asks for money. A firm no at the outset is enough.
Money changing and speciality goods. Change money at a bank or a gold shop with posted rates. For bird’s nest and one-sun squid — the two most counterfeited products here — buy from a shop that gives a receipt rather than from a beach hawker.
The thing worth remembering: nearly every trap above dissolves by asking the price before agreeing. Asking is not rude here; staying quiet and being surprised later is the awkward outcome.