I have had great success at our local coin auctions. They seem to happen once or twice a year near me and typically have 400 lots or so.
My Keys- Do your research beforehand. The auctioneer usually publishes a listing a few weeks ahead, try to find what really interests you and do your research. Also, identify FMV for each lot.
- Get to the viewing time early and make sure you visit and hold each lot you are planning on bidding on.
- Bring good notes and keep track of winning bids on all lots, this will establish "going rates"
- Find a comfortable spot where you can use your notes, etc. and be easily seen by auction staff
- Stay on target with your bids, don't get carried away. With 400 lots and perhaps 12-20 serious bidders, plenty of items for everyone.
- The local dealers are there for the big ticket items and generally will have the cash to buy them, the better deals tend to be less desirable lots.
- be prepared to stay all day. 400 lots can take the better part of the day. Sometimes the best deals come at the end when everyone is out of money.
Some Wins- Roll of 39-D
Jefferson nickels for $7, all EF or better
- Complete Jefferson BU (most Choice or better) Album for $30
- 1906
IHC MS64 for $15
- Many mislabeled lots with more value than stated
Of course, you will have your stinkers in there too. The odd thing is, I usually go looking for specific mint and proof sets, but they almost always go for above retail.

I guess I happen to live in an area where auctions are common (farm country) and everyone knows each others and it would be hard to pull a fast one.