This is something that the buyer should have done this before you went to contract. Let them have their own roofer come in and be there for the appointment. You will have to decide whether or not you want to proceed.
Land Contracts can be a good option as a seller. As a Realtor I had so many people pose this question to me directly over the years that I wrote two books on the subject available on Amazon in Kindle eBook format. One is called: Understanding Land Contract Homes: In Pursuit of the American Dream which is here: http://www.amazon.com/Understanding-Contract-Pursuit-American-ebook/dp/B00ASCTJQG/ref=sr_1_1?s=books&ie=UTF8&qid=1356626079&sr=1-1&keywords=land+contract+homes
The other is called: Land Contract Homes for Investors, which takes on the viewpoint of the seller and what you need to look out for and know about. That link is here: http://www.amazon.com/Land-Contract-Homes-Investors-ebook/dp/B00AST1Z7K/ref=sr_1_3?s=books&ie=UTF8&qid=1356629029&sr=1-3&keywords=land+contract+homes