Originally Posted by rowan

Selling large amounts (especially on a smaller exchange) is going to result in a lot of slippage, where you have to fill several orders to cover your volume, each one at a lower value. Just because the current price is $15000 doesn't mean you can sell 10000 BTC and net $150m. You might sell 50 at $15000, another 30 at $14990, another 70 at $14780 etc.

If you're not in a hurry, just sell a bit periodically, which will also help smooth over variations in value. This will help even if you're selling off relatively small amounts. If you are in a hurry, try setting a sell price around the buy price (ie you are closing the gap between the buy and sell price) which should execute reasonably promptly.

If you're a baller you may also run into problems with exchange imposed limits. The exchange I use limits fiat withdrawals to $AUD 100,000 per day (about $USD 75k). You can usually raise these limits by providing more detailed verification documents, but it may take some time for them to process your request.

Not sure if the OP ran into this sort of limit, or we're being trolled.