That's what Bitscalper was trying to be (if you pretend that it wasn't a scam).
It was a scam? I know they got 'compromised', but where is the evidence that it was them?
Plus, where on their website does it say that it is guaranteed?
Bitscalper did not guarantee against losses, and indeed losses occurred sometimes. It hasn't been definitively outed as a scam because withdrawals still rarely go through (often are canceled by server or kept for days, even weeks as "processing").
Could be viable
, but I'm not sure many would trust it after Bitscalper. It's not so much the worry about losses through bad trades as it is trusting someone just to hold the coins and pay out when requested.