I own a condo in Danvers in which I plan to put on the market on July 1, 2013. If I were to offer a lease with the option to buy, I would set my sale price higher than market value to mitigate the opportunity cost of selling my unit to another party who has sufficient funds.
Also, because inflation has been steadily increasing since 2008, I would keep the option to buy for no longer than 6 months, and then readjust according to inflation. So, not only would you pay more than market value, but you wouldn't even have a price guarantee.
So Kevin is right, it would take a lot of work to find a willing seller.
Rent-to-buys only work in the buyer's favor if the seller is 1) a relative or friend of the buyer (this is the most common case), 2) nearing the end of life and has no heirs who have rights to the property, or 3) clueless and gullible. If the seller is none of the aforementioned, then you will definitely pay more than market value for a property. This is the trade-off.