Has anyone pushed that XY button yet? Did you ask the seller to do that and if so, what happened? If pushing the button and setting the trigger to AUTO doesn't give you a trace, you know the horizonatal sweep isn't working--and that's a bad thing.
Also, do you look at eBay auctions on your phone or on a computer. I saw that damage in the eBay photos, at least the cracked back part.
You're going to have a tough time. the BK2120 was made in several iterations (not including the explicitly different A, B, versions, etc) and it is likely that what you have in front of you will not exactly match the manuals and schematics that are available.
The first thing would be to take the covers off and examine it for physical damage. Beware that although it is not an accelerated CRT, there is still a good -1500VDC at the gun and many hundreds of volts elsewhere, and then there's mains voltages in certain places.