Yes it is true!
You can sell items to a vendor without the vendor window open.
Heres how:
A - Make sure you have a keybinding for "Interact with Target".
B - Target the vendor or repair person.
C - Pick up an item from your bags.
D - Press your keybind for "Interact with Target".
I hope my instructions are clear enough for everyone
ty Khorium for spell and grammar check