I have started a new character for testing purposes. Before starting a mission i have checked the vendor, and found a very good shield for 1100 credits, but i had only the starting 1000 credit, so i thought i will sell my equipped standard shield for 110 creds and i'll have enough to buy the new one. Then i have realized i'm not able to unequip an item without replacing it with another one, thus i couldnt buy the new shield, and after doing a mission the vendor's inventory got rerolled so it was gone forever. 

My suggestion is simply let unequip items without replacing them; may it be a minor inconvinience, but still it is.