Another drawback could be liens recorded against the property that will become your problem after title transfer. Some investors who buy at trustee sales pay for a title search in advance to avoid this problem. These guys who show up to bid on the courthouse steps are professionals, and they buy foreclosures at auction as a business. They hope to buy the foreclosure at a low price to make a nice profit when they later flip the home. You do not need to hire a real estate agent to buy a foreclosure at the auction, but you do need to know what you are doing to compete with the pros.
Just remember, you will need to get the seller to agree on not only the rent to own agreement, but the terms of the agreement. i. e., length of the agreement, usually, one to two years; the percentage of the rent which gets applied to the sales price or closing costs, etc. If you get lucky, the seller may also be interested in doing Seller Financing with you. Just be sure to have a lawyer review any agreement before you sign it. A little legal cost upfront could save you thousands of dollars down the road.