The requirement, of course, is that he keep the house in spotless condition at all times. And he does, so it works out great for him, great for the Realtors, and great for the sellers.
Tip: Don't think of yourself as a renter looking for free rent. Think of yourself--present yourself--as an entrepreneur offering a low-cost (or no-cost) service that will benefit the Realtor and the seller.
Contact Realtors who handle upscale properties in your area and work something out.
Hope that helps.