Based on just the agents in my office, I would say that it is not a question of IF your home will not appraise. It's a question of WHEN.
You have some very good answers here. The bottom line is that if you need to sell, you are going to have to aggressive. I would talk with your listing agent about perhaps REQUIRING the buyer to CO-APPLY with another lender so you have TWO tracks going. Chances are your Realtor knows of a good lender that might be able to help because they have a little more structure in the appraisal department.
Below are two great links to articles about appraisal issues in the WSJ
No. 1 The buyers can raise their down payment by the difference of their current down payment and the difference in the appraisal. i.e. Selling Price $200,000 Appraisal $190,000. Buyer adds $10,000 to Down Payment.
No. 2 The Seller can take less. In the above example the seller can accept $190,000.
No. 3 Buyers and Sellers can split the difference. This does not mean they must split 50-50.
No. 4 SOMETIMES (in the past-before banking changes by this administration) the Seller can carry a note for whatever difference Buyer and Seller can agree on.
That's 4. But there are others. My list includes the most common ways to overcome a low appraisal.