Even people who change or update their phones very regularly don't have over 50 pre-owned or 'opened never used' phones to sell.
It looks as if you are trading - buying phones for the express purpose of selling them on. You should have a business account and a returns policy that allows your buyers to send their item back for any reason for a minimum of 14 days.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)