I'm getting to this one late...but to answer wallyb's questions:
1. Generally, yes, the consignment commission is the only cost to the seller. However, some auctioneers charge a separate cataloguing fee, or payment processing fee; so you need to ask those questions with each auctioneer you inquire with.
2. I personally would never in a million years sell on
ebay. They have long had a history of being biased in favour of buyers, and given the nature of unscrupulous buyers they have, I would never recommend anyone sell on that platform.
3. As for auctions, you would be wise to do your homework. Not all auctioneers are the same. When I decided to start selling some of my coins a few years ago, I inquired with each of the reputable coin auction houses we all know of in Canada, and frankly, none of them inspired me to consign with them. The commissions they charge are outrageous, and I don't find their auction sales formats inspire traditional auction-style bidding behaviours to yield maximized returns. However, I found very satisfactory success with other generalist auctioneers who don't necessarily specialize in coins and currency. That all being said, it also significantly matters about what specific coins you are considering putting up for sale, to help guide/inform your decision.
The social media auction idea coinman91 mentioned is something I hadn't heard of before; most likely because I don't use social media. I'd be open to hearing more about that...