Sim Lock iPhone XS Max

Hi guys,

Last year i bought an iPhone XS Max "UNLOCKED" from eBay, have been using the phone since 1 year in Pakistan without any issues, 2 days ago i sold the phone and reset it, when the buyer inserted his sim it shows that the phone is sim locked. After checking online, it shows the phone is Sim locked under US AT&T Activation policy, this is really surprising, if the phone was sim locked, how was i able to use it for one year?

If it was really sim locked, then i have been a fraud victim on eBay, its been more then a year so i am unable to claim it from eBay or PayPal.

I have got all the valid proofs of purchase though.

Can anyone guide what should i do now? How can i get the phone unlocked? I'm much worried... Please help!