Asking permission to have an inspection before signing a sales contract is quite unusual, but if I were the seller I would agree provided it's by a licensed, bonded inspector and that I be provided with a copy of the inspection report. That's putting the cart in front of the horse but I'd love to have a dry run on the inspection to see if there are any issues I must address.
It's usual that a contract is negotiated, and then if any adverse conditions are found by the inspector the seller must either agree to fix the issues, or sometimes an "as is" sale is negotiated with seller paying cost of repairs to buyer through escrow.
I'd love to have a free "dry run" inspection on any house I'm selling.
There is another side of the issue a seller might consider, that if any inspection shows issues the seller is not aware of, the seller would then be obligated to either fix them or disclose them in a different deal. Myself, I'd rather just fix anything discovered and have a clean disclosure.
If, however, I'm under contract and the inspection shows anything, I'd try to negotiate a seller contribution to buyer's closing costs and have the buyer deal with the repairs.
If you are a seller, nothing looks finer than a fully executed and closed HUD-1, a cashier's check, and the deal in your rear view mirror!