The best advice is that if it is a branded cellphone, then don't touch it. You run the risk of the transaction being a scam for starters, and even if the seller is legit, you will be buying a replica, no matter what they tell you.
First of all, welcome to SaleHoo! We're sorry to hear it - we do hope you have bounced back from that $20,000 loss. Thank you for sharing your experience with us. We appreciate your advice. I'm sure our members will find it very useful.
Let us know if there is anything we can do for you. All the best!