eBay has pretty decent BUYER protection.
When you get the phone obviously make sure it works and the serial is clean via Apple and identified as an iPhone X. Test every single function even if you’ll never use them (airplay, airdrop, portrait mode, slow mo, time lapse, FaceID, force touch, etc). Get an app like Lirum Info to verify it as being an X.
0 feedback is scary but it’s the reason you got that deal. As you can see from the post here most wouldn’t have bid on it. Plus it’s t-mobile which isn’t the most sought after.
But good deals aren’t impossible to come buy. I just sold a 1500 dollar exhaust system for 260 because I needed it gone in a hurry. I also bought a HomePod for 175 from someone that didn’t want to give it away but also didn’t need the money.
Almost worst case, as long as it’s an actual X and not clearly modified you can get an out of warranty replacement for $549. So you pay around full price.
Actual worse case it’s not an X and eBay can’t help you.
Good luck! Post back!
Edit: definitely record you opening the box it’s shipped in. If everything looks legit keep the recording going until you can get to settings > about. Do not let the phone leave the recording! In a worst case scenario this could save your ass!