About 75% since I have successfully sold every item I have listed at that rate
The only time I will drop below that rate is if I want to sell an item the same day or if the item in question has a fault.
Lack of places has not been an issue for me, simply listing here, ZeroIn (when it works), WeaponCrates, Prefired and UsedAirsoft gives me all the advertisement I have needed. There is also AirsoftNation but I have never bothered with it personally. If it does not sell after being listed on so many places, the item might not be as good as you originally thought.
For items which are not RIFs, multiple Facebook sales groups will see them sold quickly enough, without having to mess around with various websites, such as the aforementioned.
Too many times have I seen a sales post limited to one forum or site, expecting a quick sale.
Then again, the stuff I obtain or sell is usually desirable to begin with, which may be the reason. Sure, selling a second hand Cm16 at 75% is probably laughable, for instance.
People also underadvertise their items with a very vague description (such as "comes with 4 mags and battery, pm if interested") and crappy pictures, which impacts external interest. If you want to sell at a higher rate, such as 75%, you need to show the item's worth.
However, this is my take on second hand sales, no one else has to follow that.
Edit: Last key point about sales is keeping note of people you have previously dealt with. If they have bought an item from you once, they may want to buy another, assuming the transaction went well.
