From time to time I come across articles on this topic. I'm linking one here that is very well-written, informative and helpful.  How to Know If Your Parents’ Stuff Has Value

When faced with the job of cleaning out an estate, warehouse, storage room, attic, basement, etc., don't overlook odd-ball stuff. Don't be too quick to throw it away. For proof of how some "trash" really can become cash, check out this recent article from the New York Times. 
How Discarded Items Become Treasures

There's no doubt about it, eBay is a great resource to help with evaluating items but only if you know how to use it.  If used incorrectly, it can lead you in the wrong direction.  When searching values on eBay, you first need to know exactly what your item is called.  If you have a piece of pottery and you just search under "pottery", you will drive yourself crazy.  Thousands of results will come up.  You'll need to know the maker of the pottery first.  And then, its design, form and pattern name will also help in narrowing down your search.  Furthermore, keep in mind spelling.  Many people misspell the name or type of item they have and then get no search results.

But most importantly, once you have the correct search term, it's vital that you search first under the "sold" items section.  If you find your item(s) listed here, and listed several times, you will then have a fairly accurate guide to what it's worth.  "Sold" items are actual sales showing you the dollar amount of what the item sold for.  If you search under "current sales" then all you are seeing is an asking price or a starting price.  Sellers can ask or start an auction for whatever amount they want.  But that price is only a desired amount, not an actual amount of a sale.  And if you look in "completed" sales and see several examples of the same item with various prices but no bids, then you can disregard those figures.  Potential buyers all thought that price was too high.

Likewise, you can offer to sell your house for a million dollars but if all the other comparable houses on your street are selling in the $200,000 range, then that's what your house is worth.  It's the same on eBay. You might see several similar items with starting bid prices at $500, but if all of the same types of items have sold for $25-50, then that is what the item is worth.  So, don't be fooled by "current" sales figures.  

If you have antique or vintage furniture and you're selling it to a dealer like us, it's best not to polish or clean it.  If you're trying to sell it to an end-user, on craigslist or eBay, etc., then you may want to touch it up first to make it more appealing.  But you may be doing more harm than good by polishing antique furniture if you don't know what you're doing.  It's best to let the professionals polish furniture.  There are many products out there that can leave a film or coating on furniture and destroy the age-old patina that gives antique wood its appeal.  So, when in doubt, don't.  Leave your furniture in the state it's in.  

If you're looking to sell us your sterling silver, there's no need to have it melted down.  We buy all sterling silver in any state: broken, tarnished, dented, incomplete sets, etc.  

Do you have old sterling silver laying around?  Do you no longer use your silver flatware service?  Do you have broken candlesticks, dented bowls, monogrammed trays that no one wants?  We are actively buying any and all types of sterling silver.  No matter how small or large the quantity, we will buy it.  And we'll come to you to pick it up.  

When donating non-cash items to your favorite charity, remember that the tax deduction you get is determined by the value you place on the furniture as well as your tax bracket.  It's best to discuss this with your accountant prior to making the donation.

Keep in mind that different charities have different methods of use for the items.  The Salvation Army, for example, takes the donated items and puts them in their stores for sale.  That's how they generate money for their worthwhile programs.  Furniture Sharehouse, a Westchester County-based organization, stocks the items in a large warehouse and then distributes them to people in need.  Habitat for Humanity is another organization that will sometimes accept furniture donations.  I believe they sometimes give out the furniture to those in need and possibly also sell items in their local stores.  We will sometimes take items and give you a charitable deduction from a local charity that we work with.

As crazy as this may sound, it's not so easy to give away furniture.  Large items require skilled movers and many charities are short on man-power and trucks to pick up the items.  Or, they are so overwhelmed with donations that they may not come as soon as you call.  So schedule your pick ups far in advance of the date you need the items removed.  And, be prepared for the charity to turn you down.  Your items may be in top condition, but if they are items that are not in demand, then the charity will not take them.  

When donating items, the charity will rely on you, the donor, to put a value on the items.  You can value your items based on what you believe they are worth in the current market, not what you might have paid for the item.  Remember, if you're valuing the items at more than $5000, the IRS will want to see an appraisal from a qualified appraiser.