Bitstamp and Kraken are the only exchanges I would recommend. Considering you are all good with verifications there is no problem to sell and withdraw such amount. Of course, don't do all in 1 shot, do it during several days, better safe than nothing.
Coinbase is not so good as people like to say, and Gemini I don't know so I can't comment on it
https://localbitcoins.com is the best you can get. It has an escrow you'll get your funds first from the seller and only then you will release the escrow. However, 150
BTC is not a small amount so you'd have to make multiple trades but at the end you shouldn't face any issues. Even if you do, you can file a support case, localbitcoins is very much supportive I have heard, so give it a try.
Do not use bitfinex, if you're doing, you're doing so at your own risk. You can use coinbase too, it's the biggest crypto market for buying and selling.
There is no use for a company or the average Joe to sell 150
BTC on a marketplace like localbitcoin. it's a crazy thing to do. No one will ever buy 150
BTC and with all scammers there it's just a stupid idea. We are talking about over 20 million dollars, not a $10 banknote.