I got one in a buyout of a guy who's retiring from reloading. It was an older one, though still quite good (and Dillon will support it anyway).
It was set up for .45, and had 10mm conversion as well.
I sold it for $230. Could I have gotten more? Perhaps, but the guy who bought it did it FTF, so no shipping, which would likely have added $20 or $25 to the price (no flat rate box).
I think that price was more or less fair. Would you take a used one for $50 less than a new one? I wouldn't. $75? Still probably not. I--personally, YMMV--would want a significant discount to do a used one. So to my way of thinking, $230 was fair and reasonable.
Now, you indicate your intended one is NIB. If so, I think $300 would be an OK deal for someone who wanted a SDB, and my guess is that it's probably the max you could sell it for here. There's a certain risk people are taking in buying it from you, and my guess is that would drop the price to $300.
You also have the problem of it being set up for 9mm. I think people would be much more interested if it were set up for .40 or .45. That may limit your market too.