I worked for a few auction houses in the 1980's. They each had their normal public auctions, and tended to be lots (singles, sets, collections) that would net $100+. Very similar to what you see today at Kelleher, Seigel, or the others that we have all heard of. Each company also had a sister business of Mail Sales. Similar to an auction (collectors bid on each lot, each lot sold to the highest bidder [if it met the house-imposed minimum], and there was a closing date), but with certain differences (no roomful of bidders - all done by mail/phone/FAX, the material was more in the $10-$100 range, and perhaps only 5% of the lots were photoed).
For each of these companies, we were always wanting for Mail Sale material. We often attended auctions around the country. One of the owners of one of the companies often went to England/Ireland - he always came back with stamps, but I think he was mostly there visiting family, pubs, soccer games, etc. One of our greatest 'unknown' sources was Lippert in Detroit. We were ALWAYS there viewing early and looking through every carton, top to bottom. Once, we got a ratty old remainder lot in a few cartons, but at the very bottom of one of the cartons was a complete MNH collection of blocks of FSAT. My boss (who was there) was willing to pay $5k, based on the FSAT, and the lot opened at a couple $100. It was estimated for $300-500. It got bid up to $1500, and we got it - bidding was between my boss and a single collector. We would have made good money on it at $5k. We took out the FSAT, put the $100+ stuff in one of our auctions, the $10-$100 stuff in our Mail Sale, and sold the remaining cartons - probably untouched except for the FSAT - in one of our collection auctions. If it weren't for my boss digging to the bottom of all the cartons, this lot probably would have sold near the opening bid to the collector. I made up the #'s, but the story is true, and the numbers in my story are reasonably accurate for 'scale'.
My point is that there can be bargains to be had at many auctions (especially collection auctions), depending on the material, the market, and the audience. It CAN be worthwhile for a dealer (show dealer, eBay
seller, auctioneer, etc) to attend other companies' auctions. If they aren't there, many lots would sell for less than they do. As a consignor, one should hope that the room is crowded - whether it is full of deep-pocket collectors, or dealers, or other auctioneers, shouldn't really matter. A consignor wants his stamps to sell for as much as the room will bear, whether it is to someone else that will mark it up to get his profit or a collector who will value and admire it for years. (ETA: the 'full room' is important so that the lots are a viewed fully. Descriptions aren't always accurate. Large lots are rarely photoed in full, although that is getting better.)
As with any kind of industry, you should shop around and see which company will meet your needs best. As a consignor, that often means consigning with multiple auction houses, to see who gets the best $$$ and is responsive enough when it is time to get the $$$ into your pocket. There are A's out there, and there are F's (as well as everything in between), and your experience may be very different from the consignor that walked in 5 minutes before you.