How do you know how much you can potentially get for an item that you have picked in your home? Research is the answer.
Go onto eBay and search for the item. When it comes up you can see what others like it are going for.
Example. Grandma's old wool coat. You could put "Vintage Wool Coat" in the search bar and you will get a ton of coats to look at so narrow it down. "Vintage JP Penny's Wool Coat" and you will get a better idea of what yours could go for.
Take it a step further by scrolling down on the left of the search results screen and click on "Sold Listings" This will show you what has recently sold. Giving you a better idea of what you can get.
I have a site I go to.. http://checkaflip.com ... You put in the item at the top and click search. It shows you recently sold and currently listed items, Average price and % sold.
If you are going to do this, you have to get good at research. It will give you the current market value of each item.
Ready, Set, Research...