Check Multiple Sites
Compare prices using a variety of booking sites. The big ones are HomeAway, FlipKey, (SmarterTravel's sister site), and VRBO (owned by Homeaway). You might see some inventory overlap, but that's a good thing. You can often read reviews for a single property on two different sites and get a clearer picture of what to expect. For beach rentals in popular destinations, these three sites are fantastic.
Then there are a number of smaller, alternative booking options, including destination-specific booking sites or associations that list beach rentals. I've had positive experiences using Airbnb, but the site doesn't have a consistently robust inventory when it comes to beach towns—at least, not compared to the aforementioned big three. For more ideas, see 10 Most Valuable Alternative Booking Sites.
Beware of any listing that doesn't have reviews. Ask for references if reviews aren't available. Most importantly, never send money via wire transfer or as a "gift" through PayPal, neither of which offer any protection to the payer.