A buyer from a classifieds site

Easy

You've listed your old phone on a classifieds site. A buyer gets in touch. Chat with them and decide: is this a real buyer, or a scammer?

Reply like a normal person: ask questions, check things.
When you are ready, press a decision button.
In some scenarios it really is your bank — seeing a scammer everywhere is also a mistake.

Never type a real card number, code or password — not even here.