As you know (or will know when they freeze one of your auctions) Ebay does not permit the sale of firearms, and a LOT of firearm parts and accessories. In addition to Gunbroker.com, I also use Auctionarms.com. I have used gunsamerica.com, but not overly impressed.
The keys to being a successful seller: GOOD pictures (notice I used the plural- one pic sux) in focus, properly exposed, showing close-up what the gun looks like. A red or light green background seems to give good contrast.
Next, a GOOD description of the gun. I know that a Marlin 39A is a lever action 22. Tell me about YOUR Marlin 39A- condition, serial number prefix (tells me the year) any changes to the rifle, bore condition, etc. I can't hold it and look at it, so you gotta tell me why I should buy yours.
Last- a good price. I do not waste my time on hidden reserve auctions- list it as a basic auction, start a the lowest price you can live with, and start the auction. When I look at a price, I mentally add shipping costs, and if it is not covered by my 03 FFL, I add what an 01 Dealer will charge me for the transfer. Translation- you have a Ruger up for $240, my local dealer has for $250. BUT- to buy yours, I'll spend $240, PLUS $26 in shipping, PLUS $50 to dealer for transfer. NOT a good deal.
Go look at some of the more successful sellers on auctionarms- sportsmans supply, jack the dog- they have good success for a reason.