While buying used furniture is a good idea most of the time, there are certain times you should avoid buying it. The single most important thing you can do before buying previously furniture is to inspect it carefully for clues to whether that piece of furniture is usable and worth buying or not. Chances are you won't be able to return it, since most used furniture is sold as is and without any guarantees.

A good practice for buying furniture, new or used, is to always measure it as well as the space in your home to make sure there are no ugly surprises. You should absolutely love that piece of used furniture or know exactly what you need to do to improve it before you bring it home. And sometimes, it is best to avoid buying it altogether.

Here are some instances:

When it is Bad for Your Health

In my opinion it is best to stay away from used mattresses, especially if you don't know and trust the seller.

A lumpy, smelly or stained mattress is a definite no-no. It doesn't matter how low the price is. You may end up paying with your health as you have no idea what's made its way into it along with associated health hazards such as mold, dust mites and bed bugs. You should definitely NEVER buy a used crib mattress for an infant from a garage sale or flea market.

If you must buy a used mattress, look for a rebuilt one. While not the ideal solution, it is still better than a used mattress at a garage sale.

After buying, make sure you seal it in a plastic wrap to prevent coming in contact with whatever is inside it.

When it Has Structural Problems and Missing Parts

When you are buying used furniture, always make sure that the it is sturdy and that there are no structural defects.
If a chair has wobbly legs, check to see if it can be fixed. Cracks or stapled legs are never a good sign, and sometimes it may be hard to replace missing parts.

Also look for signs of water damage and termite or other insect infestation. Look under a piece of furniture and behind it to gauge its condition. If you think you won't be able to fix a piece of furniture by yourself, it is best to walk away from it.

When Upholstered Furniture has Been Heavily Used

It is generally better to stay away from upholstered furniture that has been heavily used. Lumps on a sofa seat or back are enough to tell you that it will be uncomfortable to sit on. Saggy pillows are difficult to fix too. It is best to stay away from such pieces.

While a worn upholstery fabric can be replaced, it can cost a lot to have it done professionally. Check the sofa frame while you're at it. Unless it is a great piece of furniture with superb lines or an antique, you are better off not buying it.

When Furniture Smells

It can be difficult to rid a piece of furniture of unpleasant smells. Both upholstery and case goods can absorb smells from many different sources such as smoke from fires or cigarettes, pets, cooking odors, urine, etc. A used bookcase I bought smelled of dog for many months and took a lot of cleaning and airing.
However, sometimes it can be impossible to get rid of these smells depending on how much the furniture has absorbed. If you find the smell unpleasant, avoid that piece of furniture especially if it is upholstered. You may end up with the rest of your room smelling like that too.

When it Costs More than it's Worth

Don't buy used furniture if you feel it is costing you more than it is worth.
That can be the case when the price tag does not reflect the condition of the piece and you end up spending even more money and time to make major repairs to it.

Sometimes the seller may misrepresent the furniture and ask for a higher price. For instance, often any old furniture is called an antique. To be an antique the furniture has to be over a hundred years old. Research buying guides for prices if you are going antiquing.

Another way to pay too much for used furniture is when transporting it drives up its initial low cost. Big and bulky pieces may warrant renting a bigger vehicle than you own. Factor that cost in before you buy.

When it Has Been Recalled By the CPSC

First of all it is illegal to sell furniture that has been recalled, unless CPSC specified repairs have been carried out. In all fairness, the seller might not even be aware that the piece has been recalled or needs any repairs.
It is best to stay away from used nursery furniture. Most cribs made before 2011 are unsafe and have resulted in injuries and deaths. Why even risk it? Check out Saferproducts.gov for lists of recalled products to make sure you are not buying one.