I dont know if this will help, but it might be worth a shot.
If you buy a used phone, and the previous owner pulls a fast one with customer service, ask customer service if there is a way you can verify the legitimate sale of the device. I would not suggest buying a used phone from online stores if you can avoid it.
One way you can prevent sellers from getting over on you, is to notify the website you used to buy the phone, that the seller is engaging in illegal behavior. In the USA, once you sell a product, unless otherwise stated in the TOS/EULA/Copyright, etc, you no longer own it. This is especially true, when you are buying a product in any condition, from any party other than a licensed retailer and/or the company who branded and/or created the product.
Some websites like Ebay and Paypal will help to protect the funds you used to purchase the device, if the seller engages in such behavior. It is best to report any incidents as soon as possible, after the problem arises. The seller may not be indemnified if they report the device stolen after the normal refund period offered by Ebay, Paypal, etc. In the USA, it is illegal to deface, or cause to be defaced, any material or non material product which you do not legally own and/or have permission from the owner to deface and/or make changes to. Any intentional or avoidable (in good faith) act/s which cause a product to no longer work as it did at the time of purchase is considered defacing, or causing to be defaced. There are no criminal liabilities here, but, if Paypal/Ebay/Swappa/etc, wont help you, then you might be able to regain your lost funds in the form of civil damages.
Please keep in mind im speaking from the point of USA laws and regulations.