blacklisted != blocked != reported lost != reported stolen...
there's different situations that warrant where it'll be blocked from and different databases. actually, checkesnfree dot com does a pretty good job explaining it, though it's not 100% foolproof. just run a test with an imei number and see the different response types. they give you a decent list of possible results.
i've run AT&T on an unlocked T-Mobile SGS3 that was reported lost and it still works. because it's blocked from T-Mobile but not blacklisted across all.
hope this gives you some insight. i nearly got ripped by an eBay seller who sold me a fraudulently obtained S5 which i promptly returned and thank God it all worked out (dude had 100% positive across several years too, it surprised the sheeeet outta me). I ended up driving to Delaware (only 40 minutes from me and I had to do other shopping anyway) and got it tax free with my friend's T-Mobile number at $660 instead of $689. paid $25 on eBay for the unlock code and now I'm 100% worry free, bootloader lock free, KNOX free lol, and rooted.
lesson: it's a $650+ investment. be smart. since I'm running it on AT&T anyway, I could have kept that eBay phone which was blocked and not blacklisted, but why bother? who knows what could happen down the line? value would also be down. plus, I made 30 minutes of phone calls with my friend's T-Mo sim card (to be safe, 30, not just 5) just to ensure it also gets region unlocked too. =)
if you do buy it somewhere else, make sure they have a receipt showing it was PAID IN FULL. cash. they could report a stolen CC number down the line too so I wouldn't trust even one paid in full with CC. even then with a cash receipt I would be careful.. they could report it stolen later down the line, collect insurance, whatever! which goes around again to this: it's not worth it, just get it straight from the carrier! it's a freakin $650-$700 device, it's not worth gambling to save a few bucks!