Just did this on my Emco awhile back and was debating to go two or three, SO glad I put one on the compound as well!
https://www.practicalmachinist.com/forum/threads/lathe-dro-install.399431/#post3885062
Here's a good resource: http://www.carbidedepot.com/formulas-insert-d.htm
You've got a lot of different style inserts there and decent holders are expensive, I'd narrow it down to a couple of types you think you'll use the most and flog the rest.
The ones near the bottom look like grooving...
If it's the bolt that's bad why not just replace it?
I uploaded a manual here: https://groups.io/g/EmcoSuper11lathes/files/V13 parts list/Maximat V13.pdf
It's called a "center bolt", D1A 050 090 on pg 16. Try Fleebay...
With that feedback rating I wouldn't be worried about a scam but as dalgirl said Ebay will refund your money pretty quick if it's not right.
If he can't just sell it to you at your price could he re-list it with a buy it now at that price?
I've made some fairly high tolerance parts from S-7 and heat treated them to 60Rc, it moves very little.
https://fordtoolsteels.com/wp-content/uploads/2017/10/LSS_S7.pdf
http://www.interlloy.com.au/our-products/tool-steel/s7-shock-resisting-tool-steel/?output=pdf...
"is it really important to get both centre drill holes spot on coaxial and parallel?"
I see your point of course, for a perfect set up the center drilled holes should be coaxial.
So the real question then is what is the definition of "spot on"?
I haven't read all the posts but am I missing the obvious here?
When a shaft is turned between centers wont it be as round/straight/true on the diameter as the machine is capable? Even if the ends are not drilled exactly in the center of the stock?