Flashing L720 back to Stock

I have come on by here a few times and at least 90% i have been given good advice on what to do or I just check the FAQ, this is a bit different as I am not sure what to do.

I work for a company that sells used phones and such. We sold a L720 to a customer who apparently didn’t like the phone and returned it back to us only they didn’t tell us that they had the phone flashed to Verizon as it now starts up with the Verizon logo instead of the Sprint logo, they also seem to have rooted the phone as well. I been trying to flash the phone back to Sprint but it doesn’t seem to be working as on ODIN it fails to flash on on Octoplus it appears to be writing but on the phone’s screen it appears to not be working as I get a lot of Secure Check Fail: System, it’s becoming a wall of text of just. I am not sure if I am doing it right but how do you flash a custom Firmware of a different provider back to the original stock firmware?

I downloaded the oct firmware from the support center and that didn’t anything other than the same messages, I tried with the MD5 file on Odin and it won’t even work just says fails after I click start. I then convert that MD5 file to OCT with octoplus but that doesn’t work, it converts just fine but still is like the same error messages.

what am I doing wrong?