The ONG connection will command a premium because it is a well documented factory M870 “riot” version of the Mark 1 (not a put together). Less than 1300 of these are out in the wild. The folders made up about half of those. While it may be uncomfortable to shoot, they are collectible.
We all paid ~$250+Xfer from Sportsman’s Warehouse a couple years ago. They sold out in just a few hours. Since then, I have seen these guns flipped for anywhere from $500 to $1200 and everywhere in between.
If you left it in the original condition you received it from Sportsman’s Warehouse (hopefully you did) it will all depend on how fast you want to sell it. I think $750-$900 is realistic if you are willing to market it properly and wait for the right buyer. The problem is that everybody knows what you actually paid just a couple years ago and will want to start negotiating from there.
ETA- Honestly, if you are willing to hold onto it and wait for the man at the end of the rainbow, I’m sure you could get $1,000+