Go to eBay's website and start shopping. Start by doing a simple search for a specific item. If it's a common item, you could see hundreds or even thousands for sale. Scroll down and look at the prices (including shipping) to get an idea of the ballpark price for the item. Note that auction-style listings may end up with a quite different price by the time the auction closes. You may be able to get a better sense of the going rate for an item by looking at Buy It Now prices.
Sort the items by price. At the top of the list on the right side of the page you'll see a "Sort by" with a drop down arrow. Click on the arrow and select "Price + Shipping: lowest first." This will sort the items and list the lowest starting bid (including shipping) or the lowest Buy It Now price. Try to get a sense of the range.
Look at the left-hand side of the page under "Refine Search" and scroll down until you come to a heading marked "Preferences." Under that category you'll see the words "Show only" with an unchecked box for "Completed listings." Check the box and click on "Completed listings." (If you haven't logged into your eBay account, you will be asked to do so at this time.) When you do this you'll see a new list of completed auctions, sorted by date with the sale price if the item has sold. If the item actually sold, the price will be shown in green. If not, it will be in red.
Sort the completed listings. As before, use the drop down arrow next to the "Sort by" box and choose "Price + Shipping: lowest first." You'll get a new list sorted by price. Scroll down until you see the first listing with the price in green. That will give you the lowest price at which the item has sold.
Refine your search if you like. On top of the list you'll see "Preferences: Completed listings." To the right, look for the words "Edit preferences." Clicking on this will open a decision box that will enable you to sort completed listings by things like distance, "Buy It Now" items, and items with free shipping.