That's the same seller I got my PSX from, but we worked on emails not eBay. He does have cheap PSXs most of the time but I got a full set of a 100% working one.
Before anything, you have the part which connects the motherboard to the harddrive ? That connects to a sub boards with blue LEDs and one of the two IR sensors the unit has. It won't power up without that board (but it does power up without a HDD, it just won't boot to XMB).