BUT IT WAS FREE! It's a real temptation, isn't it? Call it whatever: the garage sale piano, the Craig's List giveaway, anything along those lines. You spot a piano that someone is willing to part with for little or nothing. Should you or shouldn't you?
Be careful! Contrary to popular belief, pianos do not last forever. Most of the time when something is available for nothing, that's exactly what it's worth. The amount of enjoyment you will get out of an instrument is directly related to its quality, but not necessarily its price. A $100,000 piano will not give you twice the enjoyment of a $50,000 piano, for example. Make sure you spend enough to get something that will be of sufficient quality to make it enjoyable.
I'm not saying there are no amazing deals out there. Just be careful. Have a qualified technician whom you can trust inspect the piano before you buy it. The small fee he or she will charge is worth every penny. You don't want to buy someone else's garbage. Go look at the instrument by yourself first to see if it's actually something you would take home. If it looks OK to you, call the technician and meet to look at it again. He or she will inspect it inside and out, testing the tuning pins for tightness, the felt components for wear, whether it has been maintained properly, and a myriad of other things. When the inspection is complete, talk privately away from the seller, whether it's a private individual or a dealer. The technician will be able to tell you whether the piano is serviceable and also if it's worth the asking price. If the seller will not allow an expert to inspect the piano, stay clear.
Buying a piano that received the proper piano care, will hold tune and is mechanically sound is especially important for the beginning child. If the piano a child practices on every day is out of tune, he or she will learn that as "normal". This means that when the child plays an in tune piano, it will sound "wrong". Give them every opportunity to succeed. Don't fall into the trap of spending $100 on a junker at a garage sale and when the child hates playing the piano 3 weeks later, justifying your purchase by saying "I'm sure glad I didn't spend $2000 on that other one we saw".
Music is a gift for a lifetime. Few high school football stars are still playing when they are 70 years old, but the musician carries on.