The center pin is the pin that actually triggers the flash and all hotshoes (excluding Minolta/Sony) have a contact for it. The other contacts are for camera/flash communication (TTL). After looking around, it appears as if the 533G will behave like most old flash units when mounted on a Canon DSLR—it will work only in auto (thyristor) and manual mode. There have been reports of trouble getting the 533G to trigger on Canon's PowerShot G2.
I think it's best if you limit your liability by not suggesting that the flash will work with newer digital cameras. It's extra information that will have little effect on the final selling price, but it can potentially put you in a position of disadvantage should a dispute arise. Chances are, someone searching specifically for a 533G will have an idea of why they want it.