I have no idea. But when you cancelled your EPRP line, did the reps not assure you you only had 1 line left on the account?
I opened the account with an EPRP line with an area code 732 number about 3-4 days before the SERO TOL. I TOL'ed the SERO line with a California area code, and immediately after the TOL, (I mean during the TOL phone call) asked the rep on the phone to change the phone number on the SERO line. I got another CA area code number. A few days later, I cancelled the 732 EPRP number. I then put my own phone on the SERO line. I let the CA SERO line linger for a week or so, using my own phone on the line to make 1 call a day and use the internet. After a week or so I then ported my number to Sprint to the SERO line.
Maybe because I immediately changed the phone # and device on the SERO line, it really "disconnected" the SERO line from the original owner. But it worked for me.
If the seller is willing, try again, doing exactly as I did. And let the EPRP line and SERO line co-exist for a few days. When you cancel the EPRP line, make sure the rep confirms it. Then hang up and have another rep confirm. Then the next day re-confirm.
Let us know what happens.