Can't sell or buy items late game.

All day this has been happening to me. We've reached late game 30mins+ and I can't sell my items or buy new items. Everyone I click sell the item disappears then reappears one second later as though I've clicked undo but I didn't. I've lost about 3 games already because I've had to play with 5 items and a Dorans ring or 5 items and a corrupting potion. Can anyone help or has anyone else had this problem?