"We will buy your house in 10 days for an amazingly low price" is what they would say if they were honest. Honesty and scam are close to opposites.
If you really want to sell your house in a hurry, and don't care about the price you get, the fastest way is to call an auction company. Auction companies love to sell houses, and they usually do a fairly good job of getting more than you could reasonably expect on such short notice. Just be aware that the amount you can reasonably expect on short notice is a lot less than you can reasonably expect if you sell it normally by showing it to lots of people and asking a reasonable price.
If your mortgage is under water, you have no chance of selling your house fast, because your mortgage company will refuse such a low price. Even if you have equity, is it enough equity to cover your loss from selling fast at a low price? If not, you have no chance of selling it fast, no matter what you do, and there is nobody who can honestly claim to be able to help sell it fast.