If your listing isn't accepted, then it's some automated process. Try to figure out if your listing contains some banned word that isn't in the successful listings.
I'm half tempted to get a "rare 450 Marlin" barrel fitted when I send in a frame for a couple of 357 barrels this week, just to sell the 450 on ebay, but it's probably not worth the hassle. (and with my luck, I'd sell a brand new barrel for less than the cost of a brand new barrel, when other guys are selling used barrels for more than the cost of a fitted new barrel)