Although there are good deals on old flash units to be had, the electronic circuits in modern cameras will, as John says, be fried by the high trigger voltage of old but useable by remote triggering flashguns.
I have found this listing to show how high some of the trigger voltages can be
http://www.botzilla.com/photo/strobeVolts.html
The moral is definately
IF IN DOUBT CHECK.
Do not poke around inside these things without knowledge, they contain high voltages
Don