Prove you have that much Bitcoin first, and second, what's wrong with Binance?
1. I'm not the owner, I'm just the broker.
2. There are many reasons for OTC trades, especially at this volume.
- Privacy: Don't want to disclose a bunch of information to a highly trafficked and tracked entity
- Volume: You try selling 100,000 bitcoin at once on a regulated exchange. It'll either sit there for years trying to get a rapidly moving spot and/or move the market/affect the price against you.
- Regulations: Maybe you're a miner in an area with tight currency controls (ahem, China).
Those are just some off the top of my head. But there are plenty of legitimate reasons for miners and owners of bitcoin in bulk to work outside traditional exchanges.
Alright, that's not enough. If you want serious business, you need to prove you have it. And you can go OTC, OTC desks will buy up to 250k BTC no problem depending on which one you go with.