I built it up from a nightblade and crafted it during one of the Shroud of the Apocrea events awhile back. I would first have to unbind it which costs 4000 energy and with the current cost being 7,648CR for 100 energy then (4000/100)*7648=305,920CR to unbind. That's a lot just to make it tradeable. That's a lot of Jelly King runs which I get about 3kCR for each. The cost and time of building the silent nightblade, heating it, then turning it into the obsidian edge is already expensive as well as it's an event item so that'd add some worth. It's probably my favorite weapon right now. It makes Jelly King easy and the poison is the best thing ever when you keep running into healing enemies. Is it worth it to sell my obsidian edge? Would anybody even buy it at a price that the crafting, time, and cost of unbinding is actually profitable? I've never sold anything bigger than materials.
Once I get enough to unbind I would probably wait 'till I found a buyer so I don't unbind it and then have an item I can't use that nobody will pay for.
I don't know if this post fits in General Discussion but I'm not selling it yet so I didn't put it in the Bazaar section.
Fundamentally, the question is: Do you value this object more than potential buyers would value it? We can't precisely answer this question, without knowing you better.
You've offered one argument for keeping it: "It's probably my favorite weapon right now." And you've offered no arguments for selling it. So my guess is that you want to keep it.