Are they in any way affiliated with
http://BitSevens.com ? Because that site is 100% a scam. See
https://dirtyscam.com/reviews/bitsevens-com-rick-smith/(I don't understand why they would choose a name so close to the above.)
Anyway, i can't find anything about Bitseven.com themselves. They seem extremely unknown/not used. Even though it's derivative trading, (and thus there's not really any liquidity required to operate) i still wouldn't trust them with my money. They have no track record/history, which makes exit scamming a whole lot easier.
Apart from
https://bitcointalk.org/index.php?topic=4773096.0, they have no real ANN on their forum either where they can be held accountable for their service.
I am going to short btc, so are you sure that in bitseven there is no short position funding like in bitmex? This is sound unreal..
on bitseven there is no funding, it's real
So how does it work? It's a bogus market?
I think it is only coincidence that their names are so close. The link you give is not connected with BitSeven.com
I don't know how it works, but I trade on bitseven for about 3 months, and I haven't had any problem with them.
I have 2 accounts: on bitmex (about 1.2BTC) and bitseven (about 0.15 BTC). The main advantage of bitseven is that there is no overloads there. In a whole bitmex has more advantages, and bitmex still remains my main platform, but when there are overload problems on bitmex - I have the second platform when I can immideatly make trades.
Also as I know, US rezidents can't trade on bitmex platform now, but on bitseven they can trade.