Builders often list their "to be built" homes on the MLS and out on the internet as a means to attract a buyer before completion of construction. Sometimes the buyer is able to select finishes for the home which can be advantageous so that they can choose the colors and materials they want.
If you have concerns about these properties not being legitimate or have questions about them then I suggest contacting a local real estate agent to find out more about the properties.
Prudential Connecticut Realty
The reason the homes are in the MLS is another way for builders to market their product and give the public a sample of available models.
Coldwell Banker Residential Brokerage
Bethany Beach, De 19930
Just a thought....
P.S. The listings just don't get posted on these sites by the builders. they are actual listings in an MLS, put there by a member of that MLS (i.e. the listing agent). Sites like Trulia pulls listings from the different MLSs.