Clean your gun AND post plenty of GOOD, CLEAR photos. Crappy photos are an almost certain way to get a low price for your gun. The "secret" is to start it at a penny, let it run for about 2 weeks (and I believe in ending it on a Sunday night, but I haven't proven that part out), and post GREAT photos so that the buyer is confident that he knows what he's getting. Every gun I've sold this way on GB has brought at the very top of the range for whatever it was.
The other thing I love to see a seller do with a gun that I'm wanting to buy is to leave a nice scope mounted on the gun. I think this is most often done by newbies or by sellers (often consignment shops) who don't want to mess with selling the scope separately. A rifle that would normally bring $800 on GB with a scope that would bring $500 on E-bay (far and away the best place to sell a desirable scope) will typically bring about $900 on GB. I've bought several guns like this -- a Weaver or Leupold or Nikon Scope that will bring $300 - $550 on E-bay, mounted on a rifle that would bring $700 - $900, and I pay approximately the price of the naked rifle. Then I sell the scope on E-bay if it's not something that I want.
Here's a CZ that came with a Leupold that I sold for $348, and ended up with $440 in the rifle. Properly advertised, the rifle would have brought at least $800 at the time.
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Here's one that came with another Leupold that I sold for $549 and ended up with $214 in the rifle. (Shown below with a different scope that I later added, and with refinished wood. The wood was perfect, but I refinish almost all of my walnut-stocked rifles.) Properly advertised, the rifle alone would have brought about $700 at the time.
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With regard to mounted scopes, I sometimes think that a nice rifle will actually bring MORE without a scope if the scope is a cheap/low-end brand. I think it subconsciously tarnishes the image of the rifle that's being sold. I don't have proof of this, but it seems to me to be the case sometimes.