Buying a financed phone on eBay - will I have issues using it on an AT&T MVNO?

If money is owned on the phone then it must be paid off by the original owner, period. If not, the phone will eventually be put on the blacklist and you won't be able to use it on any carrier. Unfortunately, only the original owner can take care of it because AT&T won't allow you to pay off what is owed nor will they tell you who the original owner is. You will eventually have problems with it especially if you try to get it unlocked. If it were me, I'd get my money back. The seller scammed you.

I would not touch it.  Among other things, it may be restricted from being activated on any other account other than the one that is paying for it

https://www.att.com/support/article/wireless/KM1509747/

This post mentions an internal AT&T policy that took effect November 2022 that blocks unpaid phones if user on a different account.

If I’m understanding it right, all phones sold BEFORE then are unaffected? 

https://www.reddit.com/r/ATT/comments/13zobe3/does_att_blacklist_unpaid_iphones/

Would love to hear some anecdotal or policy-grounded evidence of what will happen; because everywhere I look I see both the “may be blacklisted” messages posted above and the message that “you’ll be fine to use it.”

A point, if you decide at some point in time to unlock and cannot your window to return to eBay seller has closed.

Protect yourself and return while you can, or being willing to let time runout on your discounted purchase.

You can purchase a new unit from ATT for $10.99 per month for 36 months which is less than $400 over 3 years.

https://www.att.com/buy/phones/apple-iphone-13.html?offerID=22100056&source=EC1NNBmlp0000000P&tfn=homesolutions&WT.srch=1&wtExtndSource=iphone+13&ds_eid=700000002596606&ds_cid=71700000099803245&ds_agid=58700008006361532&ds_kids=p72835850619&&&&&gclsrc=aw.ds&&gclid=CjwKCAjwivemBhBhEiwAJxNWNzCwGztrxFHnXUz3RFSQHxt_tZ4D1OYcRIs5xmpxP0X0Nvsb0FdQtBoCcLEQAvD_BwE&gclsrc=aw.ds

@brkloepfer  I would take what you read on Reddit with a grain of salt. A phone must meet the unlock requirements as listed on the AT&T unlock portal. The unlock problem that most folks encounter when attempting to unlock a phone purchased from a third party is a phone that has an unpaid balance on it, a phone that was reported as stolen or a phone and that was reported as lost and replaced by insurance. All of those may eventually get the phone blacklisted. Blacklisting doesn't happen right away but it will eventually under those three conditions. Use the phone at your own risk.

This post mentions an internal AT&T policy that took effect November 2022 that blocks unpaid phones if user on a different account.

If I’m understanding it right, all phones sold BEFORE then are unaffected? 

Supposedly phones purchased/activated before November of 2022 are not affected.  

https://www.reddit.com/r/ATT/comments/13zobe3/does_att_blacklist_unpaid_iphones/

Would love to hear some anecdotal or policy-grounded evidence of what will happen; because everywhere I look I see both the “may be blacklisted” messages posted above and the message that “you’ll be fine to use it.”

These unpaid phones are not blacklisted. They are restricted from working on any other plan other than the one that is paying for the phone.   This will affect any AT&t-owned network which would include AT&T prepaid and cricket. I have not heard of the restriction popping up on other MVNOs but an absence of evidence is not proof that it has not happened.

Phones are blacklisted for fraud or non-payment but that usually happens within the first 60 days of purchase when a customer is opened an account, purchased phones, and not made one single payment.

It is possible, but unlikely that the seller can claim the phone was lost or stolen and make an insurance claim and Asurion could blacklist the device. But that would not be normal.

Far more likely that someone lost the device, made insurance claim, found the device and figured they could sell it on eBay and make some money off it when Asurion has already blacklisted the device.  

Since you asked for an anecdote, I purchased a Google pixel 4 XL off ebay in the fall of 2019 when the phone was only a few weeks old. I paid a little less than the market value.  

    About 8 months later, all of a sudden I had no service. Top notification bar said blocked from network.  It turns out the user had not paid on his account or his phones and had defaulted on everything.  The carrier had blacklisted the device for fraud and non-payment.  Because it was 8 months later, I couldn't even make a claim with PayPal to get a refund.  

You want to buy it, go ahead and buy it. We are just telling you there are risks. There are people out there who will defraud you if they can. They don't care they already have their money, they close their eBay and pay pall account after selling a whole bunch of phones or other gadgets and make their money, and then they're gone.

As Skeeter from Texas likes to say caveat emptor, buy or beware. You have been warned, proceed at your own risk