I don't know about the US, but we also have online brokers in the UK, for new cars.
They take a small commission (usually from the dealer), then contact a bunch of nationwide dealers on your behalf. The prices they get are better than or very close to the best you could personally haggle, and it takes like 2 minutes to make the request.
Paying 2-3% more and saving a lot of hassle seems much more reasonable.