Another factor is that a number of different flash systems were developed for the Canon EOS. Some of the older systems are not compatible with newer models of camera. Does your camera manual state that the flash light should be in the Canon EX range ? if so the third party flash must support the Canon E-TTL system.
Main reason why there are so many cheap second hand flash units out there because they are not supported by the mordern camera hotshoe. The newer models that work 100% with DSLR are holding their price.
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