I heard from an agent that the New house builder's commission is really tough to get. If they can find a proof that you had visited their sales centre in the past few years before, they would reject to give the commission.
Even if they agreed to give the commission to the agent, sometimes it took the agent a year+ and many letters and phone calls to get it.
I don't think any agent would do a free buying/selling for you, and you would have to sign their buyer and seller contract, which usually state that if they cannot get the buyer/seller commission from the other party, you as their clients, are obligated to pay them out of your own pocket.
Thus, most likely, you are at risk of paying the commission anyways.