Blacklisted and carrier locked

I’m afraid there is a common scam to sell a phone, then some time later report it stolen to collect insurance on it or get a replacement from insurance. When reported stolen it is blacklisted, and getting it removed from the blacklist is difficult, and frequently impossible. You can suggest to the seller that you might report it as possible insurance fraud, which is a felony in the US.

ask for a refund

Apple puts significant protections in place, mostly for the original buyer. As you have experienced, sometimes the checkered history of a phone takes a few days to catch up. If not through Apple or an authorized reseller with a sealed box, I am not buying it because the phone may have been through 10 people of bad reputation.

Your other option is to go through T-Mobile and remove the blacklist. This is not an Apple issue.