how to find reliable seller for used iPhone online?

Sorry, but this is a user to user forum for technical questions.

You will have to search the web for companies selling used iPhones:

Did you check, if Apple Trade-In is available in your region?

Apple Trade In - Apple

Make sure to fully read through this before buying any used Apple devices:

https://discussions.apple.com/docs/DOC-250001038

It's called "The All Too Common SAD Reality of Buying a Used iPad/iPhone".

You're welcome 🙂