I have been there several times and learned a valuable lesson.
I thought I was going to save some money by buying it from Craigslist or OfferUp. I bought a phone listed as unlocked, only to find out later that it was blacklisted, even though I asked almost all the right questions.
Before buying a phone (or tablet such as an iPad or from any Android manufacturer), check the price, but be careful because these days scammers try to price their items as close to the real price as possible to calm the suspicion that "if it is too good to be true, it probably is."
Ask the seller questions. If they do not like to respond to your questions, stay away too. If they respond, ask for IMEI, carrier, owned or financed, original receipt (and warranty if possible), date of purchase, etc. Check for consistency in the answers. If there's any hint that stories are changing, it's a red flag.
Being proficient in English is not a measure of how reputable a person is.
Do not buy a store-demo phone or tablet.
Find the nearest carrier store to which the phone is locked and ask if they can assist with checking and validating the authenticity of a phone before buying. Ask the seller to meet at the carrier store. If they refuse, don't buy the phone.
Check the IMEI. Ask the seller to tell you what the IMEI is and why you are asking. If the IMEI doesn't pass as clean, don't buy it. If it passes, let the seller know that you will be performing a factory reset (this may wipe any fake IMEI and return it to its original, and scammers don't like it).
Ask the carrier to check if the phone is blacklisted, financed, or reported for any issue, including "stolen." Some carriers will not tell you this information as it is personal and may not be legally permitted.
If the carrier is cooperative, ask them to deactivate the IMEI under the seller's account; that way, it prevents the seller from later reporting the phone as stolen and asking for a replacement or claiming insurance, which will cause the IMEI to be blacklisted.
Meeting the person and inserting a SIM card to try it out is not proof that the phone is in good working condition. It may be blocked later.
Get IMEI number (if they don't know how to get the IMEI, ask them to dial *#06# on the phone, not the IMEI from the box). Here are some websites you can use to check the IMEI prior to purchasing:
- Swappa - https://swappa.com/imei
- IMEIPro https://www.imeipro.info/
- IMEICheck.net https://imeicheck.net/#imei