Owners Direct, Holiday Rentals, Villa Rentals etc. Owners of holiday homes advertise on these sites (saves paying agents fees etc). The vast majority of ads would be bona fide. I have never heard of one that went 'pear-shape.'
Remember, the holiday home owner is taking a chance also. He does not know for certain that his apartment will not be wrecked by you. Therefore, a refundable deposit might be required.
If no reply is received from an Owner, you can be sure that the place is already booked out. But, discernable owners will reply to all enquiries as there is always the possibility you might book for next year or even later this year.
There are other options like seeking out an owner in the place in which you want to have your holiday. It can be done with a phone call, knock on a door, ad in local supermarket etc. Some owners don't advertise and are not interested in a financial 'killing.' Also, some owners do not rent out their place during July and August because of the higher risk of vandalism.
I know because I am one.