The UA5 runs on 9-12v. The JB3 runs on 5v. So if it outputs the proper voltage, they should power those devices. Unfortunately, I didn't see any info on its voltage specs in my quick scan of the eBay listing. Don't forget to set the polarity properly on the UA5 power source: - pin, + ring.
As for whether it's legit - it's used, and the seller admits he hasn't tested whether it holds a charge, and if it does for how long. I wouldn't risk it, personally. There are deals to be had on batteries, but I don't think this is one of them.