How to trade 10000 BTC without crashing the market.
If you are Satoshi!
Common consensus seems to be that Satoshi (Or as we are certain: The entity(s) that appear to have mined the earliest of blocks worth many millions and has since not traded a satoshi.)
Has hodled every coin. Due to circumstances that you will never know.
But what is preventing the trading of keys. They are not your coins if you don't own the keys!
Trust is involved. Decentralized trust.
1) 10,000 BTC won't crash the market too hard, this is "only" 133 millions and Bitcoin's daily volume is 16 billions. By quickly looking at market depth, I'd say that dumping 10,000 coins would drop BTC to around $10,500 from the current $13,400.
2) Usually big sums are traded over the counter to avoid moving the market, and both sides are benefiting from it.
3) We can't know for sure if Satoshi is hodling all his coins or may sold some of them at some point, because no one really knows what addresses belong to him for sure. People speculate that he owns 1 million coins, but they don't sit in one address, most likely they are split in 50 BTC chunks of untouched miner rewards.
4) No one in their right mind would agree to buy a private key, so it's not an option for an off-chain private transactions.