All the parameters for a loan approval have changed. The big banks can cherry pick their clientele The media has made mortgage companies a scape goat in this mess, thus portraying the mortgage representative that doesn't work for a big bank as a villian.
As a realtor you want to see not only the Pre-approval but the actual finding from the automated underwriting system. This means the loan officer really did their job and ran the borrowers loan scenario through an underwriting platform. Then you want to speak with the loan officer to understand the assumptions made (down payment, taxes, insurance payment,...) for the Pre-approval. Don't just accept the pre-approval a borrower walks i with or another realtor presents.
If you don't feel comfortable with the asking these questions, ask a loan officer you work with to verify the information I've mentioned above, that way you can asure your client their home will sell, but their are no guarantees.