You can't dump Bitcoins on Bitmex 
You can only short or long them. It could go in either direction.
Check out what's Bitmex and how it works here:
https://coincodex.com/article/1960/what-is-bitmex-crypto-trading-with-up-to-100x-leverage/
You can only short or long them. It could go in either direction.Check out what's Bitmex and how it works here:
https://coincodex.com/article/1960/what-is-bitmex-crypto-trading-with-up-to-100x-leverage/
This^
And also, bitcoins being moved to an exchange doesn't automatically mean the owner is going to dump it. What if maybe he/she just put up a sell offer at $7000(or higher) or something? Dumping that much bitcoin on a single exchange isn't a good idea to start with. Jeez. Too much panic.