It is difficult to get good value for your objects. Maybe this is due to the mistake that people make that there is only one price for an object and everyone else is 'sold short'.
In reality there are many prices-the Ebay Buy It Now price, the Ebay auction price, charity shop price, bookshop price, antique shop price, the country auction price, the national auction price and so on.
Even with Ebays auctions things can go cheaper if there is a misspelling in your title, if you use the wrong keywords, a bad photo, even if you use capital letters rather than small letters-these can put people off. Even if you do nothing you could be still sold short-starting the price too low and people just watching the item and not bidding on them.
For real valuables it is best to try and get a value of the object and try and get the nearest to the price as possible. Even with cheaper valuables it is worth researching how much the item sells for.
The actual price is of course what you sell the object for.