-giving items to teammates if they are about to pick after you and you know that you most likely won't survive much longer

Okay, it makes them half an item stronger if somebody doesn't pick it up first. So does RNG, except this time it's a guaranteed item so the power gap is even less than normal RNG.

-coordinated mass selling of units when you are about to lose so your teammate can start rolling at the same time and hit some of them

Okay, so if you and a teammate are going the same build, that means you're competing with each other. Which is a bad thing. Beyond that, they're just giving a ~30 second advantage to somebody over other players that are scouting. Unless they're selling off legendary or epic tier 2s, they won't affect the champion pool significantly. If they are selling off legendary or epic tier 2s, they're probably in the top 4, so players are still going to be gaining MMR at this point in the game.