They're a 2 band selectable frequency passive equalizer. You will need some kind of 'gain makeup' amplifier in the chain if you plan on using them. Because they are 'passive', there is a 12db insertion loss when you add one to a signal chain, so if you don't have an amplifier to pick up that 12db of signal loss, you're going to kinda be in deep shit.

I wouldn't pay more than a couple hundred dollars for one...so whoever thought they were going to get over $500 on eBay...needs to up the dosage on the Prozac...