I've sold over 8000 vehicles at auctions all over the U.s., but mostly in Texas, Georgia, and the Carolinas. Most of the cars were repos, but a significant were also impounded or abandoned cars. Abandoned either on private property, or at a repair shop after the owner either couldnt afford to pay the repair bill after the car was fixed, or the repairs were more than they could afford and never had it fixed in the first place.
I will say, I have let some very nice vehicles go for a lot less than you'd ever get them anywhere else, and I have also dumped some seriously bad cars. There is a lot of slime in the auction business. There are also a lot of good people in it too. It didnt take long to figure out who was who. I worked for a very respected company and we went out of our way to disclose anything we knew about the cars we were selling, i.e., known previous body repairs or frame damage, modifications, aftermarket parts, recent repairs, etc, and had the auctioneer announce it before bidding started. Not everybody does this..... Some things are required to be announced, like inoperative odometers, frame damage, salvage titles, whatever. Other things arent.
In general, if it was the first time I had run a fairly decent car through an auction, if I was getting 75% of the 'average' Black Book value, I sold it. On the second run, if I got at least 50%, I let it go. The more you run it through, the lower the bids go. They rarely went up. Sometimes, but not often, but on some of them I had to take the chance. many times the same people show up every week, and they get spooked seeing the same cars week after week.
About 10 years ago we had a BMW that had fire damage, and it sat at the auction for weeks. No one bid on it, week after week. Under the law of the state this was in, I had to sell at auction, in order to collect the balance on the loan it had on it. No one would bid on this burned hulk of metal. I finally approached one of the dealer buyers standing around and offered him $100 to buy it for $100. As a favor he did it, and he sold the scrap for a few bucks. If I hadnt done that, that stupid car would still be there, I have no doubt.
Its hard to give advice on which auctions are better than others, because you will find a mix of good and bad at just about all of them. As for the cars, in general, my repo cars were the better bet, followed by the cars we rescued out of repair shops. Impounds were hit and miss, for various reasons I would avoid them. Most of our good cars went through the dealer-only lanes. The lesser ones went through the public lanes. The dealer buyers knew what to look for, and only bought cars they knew they would make money on. The public was.... less informed. They werent as savy, they bought on emotion, and would pay more for stuff that just wasnt worth the dealers time to mess with.
Most auctions will not let you drive the cars beforehand, but most of them would let you show up early and crawl all over any car you are interested in. Check the fluids, crawl under it, start it up, put it in gear, etc. Then bid accordingly.