You took the tag off. There is no tag on the bag. You no longer have the tag. You have no receipt. How are they supposed to know the bag is from NM without one of those items? Also if you paid with cash, sorry, you need to cut your losses and sell it.
Even with Nordies, you would need the tag or receipt or the card you bought it with. NM isn't return when you want, they do have a time limit unlike Nordies. However sometimes they will except the return if you ask nicely. But without any proof you bought it there, you need to put it on ebay.
I try to keep recepits and like stuff together. I thought I was going to keep it but I have since changed my mind. I paid with a check. I can find a copy of the check. What can I say, I was in love with the bag - now I'm not.