1756-series I/O modules don't come with wiring arms or wiring arm hoods: you always purchase them separately.
The 1756-OX8I needs a 1756-TBCH (Screw/Cage Clamp) or 1756-TB6SH (Spring clamp) wiring arm. These come with a hood and door assembly that slides over the top and the whole thing plugs into and locks onto the ControlLogix I/O module. You can get an extended depth hood if you use heavy-gauge wire.
Since it's eBay, it's possible that these were removed from service and they'll throw in the wiring arm, but if you get one without the wiring arm you are getting the same thing you'd get from the factory.