I think this business takes a gambler's mentality. Once you learn to "count the cards" and fold when appropriate, the odds are in your favor.
That's great advice. Sometimes the best tactic is to know when not to buy, vs knowing when to buy.
As far as online vs offline, I've personally been burned more than once by online auctioneers taking misleading photos of units. There was one that was 10x30 full of boxes. They took pictures inside of 10 boxes, all of which had decent household items. I bought the unit and was shocked to discover that the remaining boxes (and there were like 100) were ALL full of paper documents. It would have been clearly obvious to the auctioneer who took the photos, but they chose not to include those in the online auction.
Another time, I got burned because I couldn't smell through the internet. While the photos looked good, everything in the unit smelled like it had been part of a homeless camp - dank, cigarette smoke, urine, feces. I don't know, it was just disgusting.
Those are just a few possibilities when it comes to online auctions. I have gotten good at paying attention to the details. If say, there's a close up of a nice looking dresser in one photo, I'll find that dresser from a different angle in another photo and see that, oh - it's missing a drawer, or the side is all jacked up, etc.
If you're just starting out, go to live auctions with no money and just watch and learn for at least a month. Get to know people. Find out how they did on a unit. Eventually you'll start to see what the pro's are buying vs what the newbies are buying.