I just read an identical post on this subreddit.
Can't unlock the phone because it has an unpaid balance, as told by two separate T-Mobile representatives. It is very possible it will be blacklisted at some point as well.
From the seller,
"We’ll review the IMEI and the reported T-Mobile balance issue further. Thank you for your patience while this is being checked."
4 weeks later no reply and I ask for a resolution
Seller ,(B2B MOBILE AUCTION)
"Thank you for reaching out. We’re sorry to hear about the issue and apologize for the inconvenience.
Unfortunately, since this order is well beyond the 30-day return period, it is no longer eligible for return or refund. The previous return label is also no longer valid or available.
Thank you for your understanding"
Yup. I understand sellers are forced to follow the site rules with no consequences.
I'm extra annoyed because it's my 81 year old mother who got screwed. And I'm partly responsible because I suggested Swappa.
If I get a resolution I'll update the post but I'm not hopeful.