The MB connectors are on the bottom right. I think this has been linked to before, possibly by the eBayer himself. The part that has the MB connectors and the part with the switch itself are 2 separate pieces. They connect through the male/female (what looks to be db-9) connectors that are shown on the top left & right. Though there doesn't seem to be an insert included, you'd mount the one on the MB connector cable piece onto one of the PCI slot dust panel thingies on the back of the case.
The description states that you can use the power connector to wire up your coin door lights or whatever. Since the smaller cable which connects to the MB & a power connector stays IN the case, you've now got a power connector outside the case as well. Not really any different than lengthening the wires on an existing one, but it gets the job done.
I'm pretty positive that this was not designed specifically for a MAME machine. I'd be curious to know what the original application was.
It's not a bad little setup, but it sure ain't worth $35.00.