I wouldn't be nervous solely based upon an FHA loan or low down. The current climate in lending focuses on documented employment with history.
While you have the right to ask for a credit report, you might also be giving a buyer a signal that you will be difficult to deal with. Few buyers are going to feel comfortable sharing their data, and especially if the practice is uncommon.
Whether the preapproval is for an FHA or conventional, either way.........the buyer could be denied their loan. There are plenty of people with 20% down that lack sufficiency in documentation who fail to qualify. As a reactive society, our lenders are now concerned with documentation in this post bubble world.
Is the property that you are selling in good condition? Is it a condo? FHA will require a property inspection in addition to the buyer meeting the lending qualifications. Many condos are currently not FHA qualified.
Assuming your property will meet FHA guidelines, here is my suggestion. Ask you agent to phone the loan officer who wrote the preapproval. That loan officer cannot disclose confidential info, but a conversation with him/her may give good insight. Your agent can ask if the buyers have submitted documentation in it's entirety or only partially. A good question to ask is if the preapproval based only upon a phone call and credit report? If your confidence is lacking at that point, what can be done to solidify it? i.e. Maybe the buyers can submit their docs? If the buyer's lender is working to help the buyer secure the contract, the lender will guide their buyer to submit docs, etc.
There's never a guarantee that a loan will be approved, and it's very rare that we see a mortgage contingency waived. Rely upon your agent for some due diligence with the loan officer....but don't lose the sale solely on the fact it's an FHA loan. There are hundreds and hundreds of those that are making it to the closing table right now.