Well, by selling through GB or auctionarms.com, you're paying them to let you auction your stuff. If you're not sure of the value, you at least have to come up with a minimum bid, if not a reserve.
Let say you set your reserve at $1,500, and you only get one bid for $1,500. The pistol is sold, and you have to pay GB/AA what will amount somewhere around $100. so you netted $1400.
why not just put it up for sale for $1400 in the first place?
My opinion is that the auction sites are for when you know what it's worth, and either you can make money for selling below current value (people buy stuff just becuase the price is good), or the demand is such that it'll move regardles of price. The auction sites aren't the place to try to get the most out of it you can.
you're better off researching why a 96% Ithaca would bring $4K.
I think you should decide what you think would be a fair price to sell it for and post it for sale at that price. It might be a deal to someone and they snatch it up, or if it's too high, no one will buy. So then, after a week or so, you reduce the price about 5-10% and try again
If you're going to do the auction sites, know what makes you gun valuable and post it in the ad. If it was built in a year of low production, say it. If it has something unique, like the Ithaca Mfg., thne let it be known why that makes a difference in it over a same year production colt. Not everyone knows, but they may have money burning a hole in their pocket.