Legal Question in Construction Law in California

First, questions of this sort really require a two-part answer. The answer to any question beginning "can I be sued" is almost always "YES" and if you are, you'll have to bear the costs and nuisance of putting up a defense. All it takes to sue someone is to cobble together a halfway readable complaint and summons, pay the (now rather significant) filing fee, and get it served on the defendant. Pretty easy.

The real questions are (a) what will it cost me to put up a defense, and (b) what are my chances of winning? A third question might be (c) If I should, perchance, lose, how much will I have to pay in damages (and maybe, how much of that will my insurance cover)?

Sure, you can be sued, and if the plaintiff is a licensed real estate broker, the chances are he's no dummy, and also he's probably hired a smart attorney. The additional facts you've given regarding charges and purchase of materials seem to be irrelevant.

You need to defend the lawsuit, if and when filed. You may win, you may lose. The chances of losing are close to 100% if you fail to respond to any lawsuit, if and when filed.