Yes, most brokers source their deals. Not sure why they would hold back half of them. There are pre-market deals that some brokers will pre-shop to 'insiders'. Let's them test interest and pricing before going to market. By SBA I assume you mean sub-$5M deals? SBA is a lending program.

Yes. A sizeable portion of deals happens through the network. You know a guy who knows a guy. The moment people know you are looking for one, leads get routed to you. Also, once people know someone wants to sell, there are people willing to invest if it is a good deal. Broker deals, especially BizBuySell leads, are ones that most likely went through a network that wasn't strong enough for someone to pick up, and then those languish for a long time until a search funder picks them up, since he doesn't have a network yet. Good deals do come from the Broker list, but brokers do have a preferred list, so it will be offered to them first. What doesn't get picked up, then gets passed around. At least this has been my experience.