Buying a second hand musical instrument is a something almost all musicians do at least once in their lives.
Physical Inspection
When inspecting a brass instrument for possible purchase, look for the following telltale signs:
General appearance:
Check for dents. All dents will affect the intonation of an instrument. The bigger the dent, the more serious the potential intonation problem. Check for pitting, especially around the valve casings. This is usually the result of heavy use by players with excessive acid in their perspiration. Check the water key for leakage. This key may be misaligned, or the cork may need replacing.
Look for small pink dots (actually copper residue) inside the leadpipe or the slides. This is called red rot, and is the result of the dezincification of brass. It is evidence of serious neglect. Once dezincification has begun it is impossible to stop. LEGATO INTERNASIONAL DOES NOT SELL SUCH INSTRUMENTS!
Valves
Take out the valves and look at them closely for signs of wear. A poor playing habit is to press the valves at a slight angle rather than straight down. This can damage the coating on the valves, and is usually evidenced by a heavy vertical line.
Do the valves bounce when pressed and then released? This indicates that the springs need to be replaced. Do the valves "wriggle" inside the casing? This is an even worse sign of excessive wear, and suggests that the instrument may give serious trouble. LEGATO INTERNASIONAL DOES NOT SELL SUCH INSTRUMENTS!
Slides
Check that all slides can be removed and replaced freely.
Check the slide and valve compression by pulling out a valve slide, then depressing the corresponding valve and listening for the 'pop' as the air pressure equalises. The absence of such a sound indicates that the air seal has been compromised. [This is suggested as a test only. Do not make a habit of doing it. Excessive 'popping' can damage the instrument.]
On a trombone, check for slide compression by placing one thumb over one end of the tube openings and the thumb of the other hand on the other opening. If the slide moves, then the compression is weak.
The slide should move freely. Poor alignment is also indicated by well-worn spots on the slide.