You may know all this, but most don't so I'll use it as a learning opportunity for the rest:
A Contract for Deed gives the Buyer legal rights to the property beyond "rent to own" or simply renting. A CD is similar to a mortgage in many respects. There are differences but I don't want to go through all the nuances.
The biggest thing is that if you have a current mortgage, selling your home as Contract for Deed typically causes the "due on sale" clause of your mortgage to come due, which allows the mortgage company to demand immediate payment in full.... OUCH!
Buyers are nervouse buying a CD on a house with an existing mortgage since if the seller doesn't pay their mortgage, the new buyer could wind up losing the house when the bank (which is in 1st lien position) forecloses on the house... also an OUCH.
If you are "free and clear" on the house, then to answer your question, the rate is largely dependent on terms such as down payment, length of contract, credit history of the borrower, purchase price, etc. 1% or more above market rate is not unreasonable in many circumstances.
http://www.mnhomescontractfordeed.com for more buying or selling cd information