I generally do not want to do a contingency unless the property to sell is already under contract and waiting to close. The buyer agent would have to provide the signed contract for the home to sell, the buyers pre-approval, etc.
It also baffles me why a buyer would want to do a contingency without the home to sell sold, new listings are rapidly popping up, price reductions, aggressively priced REOs, etc. It only makes sense if the home is very desirable and hard to come by, but then the odds of the seller doing it are even lower.
Why would a buyer want to lock in terms so early on? There is a good chance the seller would give better terms if they could offer a quick closing, and less of a chance they are out due diligence money.