Many older pianos will require some repairs in order to render them playable.

There are many degradable materials used in piano manufacture. Metal springs, hammer or damper, break easily after a time, loops perish, cotton and felt deteriorate and leather becomes brittle or friable. All of these materials can easily be replaced, though if there is much deterioration, action renovation would be necessary, at least of the parts that have failed. This is rarely cost-effective, so unless the piano is of a sufficient quality or perhaps of sentimental value, I would not suggest it. When I was starting out in the 1980s, many, usually decent, but not necessarily well-known or valuable pianos were sold by dealers, rebuilt, but this is not the case nowadays, as newer pianos are to be bought a lot more cheaply.

The most common, serious fault is that of tuning pins which have become loose on account of the wrestplank, into which they are set, having dried out. In some cases, the pins can be tapped in gently, or if not, replaced with a bushing and/or a larger pin, but this is not always successful and also a not particularly desirable state of affairs. Occasionally, this condition can render a piano untuneable and therefore useless.

Strings, if broken, can be replaced; wound bass strings can either be remade or replaced with a Hexacore string, whose copper winding can be stripped to the desired dimension.

Wooden parts, if broken, sometimes from becoming brittle, can be replaced or in some cases, re-fashioned. Pianos are, or were, put together using hot glue, so that any joint can be disassembled. Action parts are sometimes fairly standard, sometimes not. 

Moving action parts are pivoted on a soft metal pin through a felt bushing. These bushings can often become worn, resulting in the hammers "flapping about" and a generally noisy action. Any slight wear should be eliminated by re-pinning, as, otherwise, the problem gets worse.

Occasionally, I come across a cracked frame. This might not affect the viability of the piano, or it might; but it is not really a repairable defect. More common is a compromised soundboard and/or bridge, resulting in unpleasant noise or vibration, a defect which is sometimes possible to remedy.

These are just some of the defects which I come across on a daily basis. Suffice to say that nearly all pianos require some repair (as opposed to routine maintenance) which can usually be dealt with reasonably quickly and easily.