Roman Republic L. Cassius Longinus, Denarius

and congrats on a nice denarius.
I seriously considered getting on of the types a couple of months ago, and it's still on my list.

like most other coins.

TIF is your best bet when it comes to auctions.

Numisbids is just a site that shows you most of the upcoming auctions. Sixbid is a similar site.

With both, in many cases you can submit your bid to the auction house via the third party (Numisbids or Sixbids). I've done that twice. Once successfully, and once with a problem. The problem was that it appeared that my bid was successfully submitted to [whatever European auction house it was] when in fact, the bid was rejected without explanation. I did not know it was rejected until after my lot had left the block. It didn't matter because my bid would have been too low.

Whoops, I'm getting off track. Here are the basics:

1. You can submit your bid directly through one of those aggregators (Numisbids, Sixbid). They will submit your bid to the auction house. You will still have to finish your transaction through the auction house if you are the winning bidder, or at least that is how it worked the one time I bid this way.

2. My opinion: it is better to use Numisbids and Sixbid to track coins you may want to buy. But, do your actual bidding on the auction house's site. You have to register with each such firm. It's not a big deal, just the usual name, address, email, password, plus references (other auction firms/coin sellers with whom you've transacted business). I doubt they actually check though because often I've registered and received near-instant approval. Some auction houses require you to state your requested credit; I bet that if you request a high amount they might check your references. I've never requested more than 10k EUR credit and that is well above any amount I plan to spend at an auction.

3. Many (most?) auction houses have live online bidding. Warning: many use Coretech, a Java-based auction platform and it is very buggy. Read Coretech's instructions carefully about modifying the settings or it may not work. Make sure to update JAVA.

4. When you submit a bid in advance of the auction, you are submitting a "secret maximum bid". For instance, if the required opening bid for a coin is $100 and you submit an absentee bid of $200, when the auction opens if there are no other bidders you will be the high bidder at $100. If someone else then bids and bumps it to $110, your bid will automatically be increased to $120 or the next bidding increment... and so on. If the bidding goes higher than $200, you don't win the coin. If it hammers at $160, you only pay $160 (plus the buyer's premium)

5. Buyer's Premium: the auction house surcharge. They range from 12% to ~22%. Read the auction house's Terms and Conditions carefully. Shipping is a separate charge.

6. In general, I believe it is better to bid live. I see nothing to be gained from bidding ahead of time although this subject has been debated here several times and there are valid points to be made on both sides.

Edited to add:

I have bid with CNG, Goldberg, Roma Numismatics, Stack's Bowers, Art Coins Roma, Hirsch, Gorny & Mosch, Pecunem/Gitbud-Naumann, Heritage, CGB, Dorotheum, and maybe a few others. If you have any specific questions about one of those, let me know.