Matthew, like @Brad T. says, it maybe a good deal. In some markets the housing supply is becoming tighter as it is here in the Denver metro area. While that is not the main reason to purchase the property you maybe able to increase the rents making it cash flow better. Sometimes with low vacancy rates rents tend to be under market value, so you are able to increase the rents as you have turn over or with your month-2-month tenants. Additionally, you maybe able to reduce expenses which will also increase your cash flow. The important thing is to value the property as it stands today not only based upon CAP, but also CoC and the market value.
I have a duplex that when I purchased it about 7 years ago, and have gradually raised the rents from $900 to $1150 and market is $1250. I have recently refinanced the property to also help the cash flow property by reducing the interest rate by almost 2%. My vacancy remains low because I am able to keep the rents under market and I offer discounts for well qualified renters if they sign longer term leases.
You need to know what is acceptable for you; this should be part of your business plan and exit strategies. Even though you expect to hold long term; when the time is right you may want to cash out (1031) and move to purchasing more properties or even get out altogether.
Good luck with the deal and let me know if you pass on this opportunity; I maybe interested;-) Remember successful people refuse to fail!