GOLD and below: Why don't more junglers buy sight stone and trade it in late game?

I if you were to get a little ahead you would have space for and the extra vision would be oppressive. You could always trade it when you needed the slot later in the game. The trinkets are nice and pinks add 5 more wards (like everyone is going to buy one.) 800 Is too much even when you are ahead. Seems like it, plus a little from your friends, would shut any counter jungling down.

Corrections?