This is a new idea to me also and I also have had some concerns. I've communicated via email with a couple of folks from this board about rentals - most have been very nice and helpful.
Everyone has different policies too. (policies on how they handle making the reservations, payments, cancellation policies, etc...) You need to come up with a "policy" that you are comfortable with and then find the right match for you.
Personally, I've done searches for "names" here on the Dis to get a "feel" for how that person conducts themselves on these boards. Many have a long history here. Others are very new (just a couple of posts, etc...) and I must admit, that makes me more nervous.
I have also run across one or two who were willing to write up agreements between us for the rental. That way , we both have a signed piece of paper that states a transaction was made and there aren't any misunderstandings about the arrangement. That might be the "business person" in me , but I feel a lot more comfortable with it that way. However, I am sure that there are many out there that would feel that a written agreement is a "hassle".
Good luck to you in finding a match!