You can never eliminate all risk of a foreign rental, of course, other than by staying home.
You can reduce risk by dealing with a large and trusted middleman (such as Airbnb) and with a provider that has a lot of positive feedback (whether literally internet-style feedback or reputation with a traditional rental agency). You see where I'm going here- reputation is what will reduce the risk.
You can usually ensure you won't lose any significant money directly to a crook (say by using Airbnb and a credit card, which provides two levels of protection), but there is a bit higher risk that what you expected won't be there. I have never dealt with 'Roomster' but I did have a terrible experience with another website (which I won't name) which left me in a strange city at midnight with no accommodation- and they added insult to injury by refusing to refund their booking fee- they did an e-mail apology, big deal).
I would suggest you might want to avoid Craigslist if you're risk-averse. There have been a few cases recently of people renting legitimately on Airbnb and then advertising the space at an attractive rent- showing the place and taking deposits from people, and then when the poor folks arrive with their belongings (sometimes a truckload) they find the place is occupied and unavailable (and, probably the least of their worries, their deposit of a few thousand dollars or whatever is long gone). It would be even easier to pull such a scam with an overseas renter.