cannot sell in-game shop items

So I've had this problem since I started playing this game, that I cannot sell any item in the in-game shop (and yes, to answer the basic question, I try to do it at the spawn area) and it gets really frustrating when my takes up my 6th item slot, so I cannot finish completing my build. the big SELL button in the shop is always grayed out, even if I highlight the item, left click it etc. But I can undo my purchases to clear that out. If you need any more info I'll try to provide as much as I can. Thanks