MMB on selling screen item will pull up tech tree sadly. For items not in tech tree (purchasable ones) bootypedia will show up. To force bootypedia every time you gotta do
CTRL+MMB.
Funnily enough I was instantly annoyed by the same thing 3 months ago when I started playing (currently at 71% progress, geez christ).
Here you can learn why tech-tree got priority over bootypedia when MMB is used (which will become a lasting minor annoyance).