Nope! This is a big scam going around ebay now and buyer due diligence is not helpful. The blocking occurs long after you have left positive feedback for tbe seller. As for swappa they check IMEIs at the time of listing, the blocks happens a few months or even up to a year later.
There is no evidence that any of these entities facilitating the reselling of phones have any mechanism, or is any possible, to stop this fraud.
And fraud is what it is, that CL seller may have told you one story, but the reality is hordes of phones are being purchased from carriers under 2 year installment plans with stolen identities. You can't blame t mobile for trying to stop that, but I do blame ebay, craigslist, amazon, swappa and others for not doing a lot more to bring this problem into the light.
I found out before I got scammed, ended up buying a new sealed phone from walmart, downgraded from the high end "like new" one I wanted from ebay.
Note that even though my search was refined to only NEW items on ebay, virtually none of them were actually new in sealed box. Look at amazon results/offers too. And "unlocked" absolutely guarantees it is an opened (actually used) item.