I would recommend going through a broker. With a broker, they will guarantee your money (but still not your reservation - there isn't any way to do that). That should help put your mind at ease. If that is still too much risk, renting isn't for you.
There are ways to know if a DVC member is legit, but every thing that you do can be easily countered by someone intent on scamming you - and every step you take can also have an innocent explanation of why it doesn't work. For instance, some people will tell you to check with the county for property registration, but if you are working with a wife who has a different last name than her husband, and DVC is registered in his name, you may not find it. Or, you might have a scammer. Its isn't too hard to steal the identity of a real DVC owner.
Even if you have a real DVC owner, yes, they can cancel at any time. And DVC can also cancel your reservation if the owner hasn't stayed current on their dues.
It isn't a good solution for worriers. Scamming is rare, but sleepless nights for those of us with anxiety over such things are not.