I don't even know if this happens between professional numismatists, I had only heard this in some distant past conversation. I know the practice is fairly common between corresponding collector friends. I'm unaware of any item in the bidding contract that would prohibit this unless there are state and federal laws prohibiting the practice.
It is called collusion and it is not necessarily illegal. You can decide for yourself whether it is ethical. There’s lots of shades of gray in the world.
This assuredly does not happen at the middle to top level, not now and not in the past. I know many of the top dealers and agents, and their livelihood depends on strong prices in the short term for their commissions, and strong prices in the long term as everyone is eventually a seller. It did happen in the fine arts world (Christie's, Sotheby's) but the other way round, cooperation to raise not lower prices, and there were major legal consequences. Low level vest pocket dealers or collectors may at times say to each other "I've limited resources and am thinking of going for a,b,c" and the other reply "good to know and good luck, I won't be on those" but that assuredly isn't "meeting, identifying overlaps, citing maximums and agreeing not to compete" as quid pro quo Today's frothy auction results anyways show that if anyone thinks there's any form of agreements to lower prices, those involved are clearly woefully incompetent as results have never been stronger
I only used an agent once when I could not participate in an auction directly. I dont consider the 5% fee too high but I dont know any trustworthy agent based in Europe personally (I live in Belgium) and I feel that getting one based in US would make it complicated regarding the shipping, conversion etc.
I endorse all the positive comments made about auction representation. It costs less than a bid increment and may save you several increments lower or result in you bidding higher or not bidding at all or identifying a coin as tooled or fake. And there's a real service provided. You end up buying coins worth 50% more than you would have bid without agent because their comments make you bid two increments more, or not buying coins that would have been worth 50% less Agents are the cheapest possible numismatic service you can buy
Martina Dieterle, based in the Black Forest is super highly recommended by me if using agents for the first time, be aware and respectful about the scale of your business to them - an agent won't travel 500 miles each way with hotels to inspect then bid on one coin at $500 for a possible $25 fee. However an agent already at an event such as Coinex or NYINC may be much happier to take on a few low level bids to add to their list