I'm a former multifamily broker who only represented sellers, which is the same as Marcus & Millichap's strategy. Good brokers at M&M do a great job of driving a marketing process and getting buyers to compete, but not all brokers at M&M do a great job.
Honestly, your broker is not wrong in saying that if you put a desirable property on LoopNet and CoStar it's going to get tremendous interest. Understanding that you want a national search, do you think he'll be able to find you more deals in the future?
Remember, he walked you through the process and helped find you a deal that's going to make you a lot of money. It often pays over the long run to be loyal and give that same broker the first crack at selling the deal. If he doesn't get you your price, then go hire a bigger firm. Good young brokers grow with their clients.
Finally, the national firms don't care about buyers; they make their money representing sellers. So if you're looking for someone to provide "consulting" on the buy side, talk to your regional guy about a retainer fee for the acquisition so they aren't dipping into the selling brokers' fees.