Buying a used enlarging timer can be disappointing if the person selling it knows little about how an enlarging timer functions and tells you that "it works" simply because it can be turned on and the pointer moves.  An enlarging timer is supposed to turn off an enlarger after the time you have set on it has elapsed. It must be accurate or it is worthless. Ask the seller to set it on a certain time, like 5 seconds, and push the start button, while comparing the elapsed time to that of the second hand of a clock or watch. Does it measure time accurately? It should be repeatable. It should measure accurately every time, not gain or lose a few seconds during successive exposures. Also a timer should do its job quietly. Of course some timers can be set to buzz to indicate when the time is up, but they should not make buzzing or grinding noises while they are timing an exposure. Those noises indicate that the timer is probably on its last legs and won't give you good service. It needs to do its job accurately, quietly, and dependably time after time.