Did the seller gave you a CD or something containing at least 2 files called "nand.bin and cpukey.txt"?
If so then just follow
this tutorial.
To set up FSD in a more noob friendly way (without horizon and other tricks), follow
this tutorial.
Is it possible for you to hook up your 360 via AV cables instead of HDMI?
If so, try booting it with the eject button again.
From what it seems, it does boot to XeLL (which is what you need to flash your NAND and obtain the CPU key)
The only real way to check if it's really a JTAG is to strip it to the motherboard.
Then with the picture I provided earlier, you should be able to see some resistors.
If you don't see any resistors near there, it's a stock 360 and you've been ripped off unfortunately.
I'd say, make some high resolution and high quality pictures of the front and back of your 360's motherboard and post them here.
Small blurry pictures are no good.