The option comes up and the list appears just like any other trader, but the items to buy aren’t clickable? Do I have to complete the whole questline or is it a graphics bug?

Try clicking all the way to the right, on the prices for the upgrades.

I remember that I had to click slightly off of what I wanted to get things. I think it was just to the right and up from each price. So, float your mouse around till it looks like you can click.

Since this might result in you buying something you don't want, save first.

Thats great guys. Muchos gracias