On PC, when I try to buy stuff from a ship trader or nanite blueprint dealer, their menu shows precisely six items. It won’t scroll down. However, if I buy all of an item, so the slot is empty, the next (seventh) item will then scroll up.
So there are more than six items available - I just can’t scroll down to them.
Same here on ps4! 
I have to push down on the D-pad with a controller to scroll down. So it is different from before. Not sure what it is on keyboard.
On PS4 move the curser onto the up/down strip arrow and then use directional buttons.
Note: not all have a second page and some tech venders and trading post NPCs can’t be scrolled. It’s take it or leave it apparently.
There are letters next to the scroll bar. Tap the accompanying letter on the keyboard.
But yeah, it’s silly you can’t just scroll.
I know. I have tapped them. Nothing happens. Some of the menus just don’t scroll.
Yeah I think some of the new menus just add new things as you buy stuff I don’t really like it because I know there’s more multi tool blueprints only have like eight but all the ones at the vendor are grayed out cuz I have them and cant scroll anymore