Hmmm. If the seller is telling you it has a bad ESN, even though it passed our ESN Check, I guess I'd err on the side of caution and not buy it. Only because maybe he means... it WILL have a bad esn because the balance won't be paid off, or whatever... we can only tell you the status of the ESN *at that moment* - maybe he gave you the wrong ESN?
If he gave you the MEID (starts with an A), ask him for the HEX or DEC and check that one instead. Ask him for the ESN from inside the phone's software, not the one on the back of the phone under the battery - the one inside the software is 100% accurate, the label on the back could have been modified.
I'd proceed with caution. If you do not have unlimited access to our ESN Checks, please email me at email@example.com with the HEX or DEC (mention you're penmaker from the forums) and I'll do additional checks for you free of charge as a courtesy.