If you have too many individual items in your inventory, you cannot sell items, as it will cause the game to crash. The solution seems to be to open a custom map editor and sell the items in the individual tabs, however this can have some serious slowdown as well, especially in the portals/bases tab. This is still an issue in the main menu even when selecting a different quantity of items (for example if I have 20,000 road tiles, selling 1, 19,000, or all of them still causes the crash).