I think the one over riding criteria is finding a treasure - period. If, you purchase a piece at a major auction, that is correctly described and sells at the top of it's value, I see little chance of making money if resold later. This was not true of Debbie Reynolds who, for the most part, held her pieces for decades before cashing out in a hot market with a major museum, well financed, in the works. That is the exception.
You must find barn fresh pieces that are important and buy them for less than they are now worth. Presented well, with images and documentation, will assure you get a fine return on your money. I continue to do this, with limited success, although it has been my sole means for 20 years now. I feel I am lucky to find one or two good pieces a year that are "steals" and will make a profit. But, as with intangibles, you must be prepared to lose as well and that it all comes out in the end with gainers outpacing losers. My one failing with many pieces is that I sold too quickly and forfeited profits. I have seen pieces of mine resell a few years later for double and triple what they achieved for me. Certainly, I used the funds to purchase other good pieces, but it makes me regret I was in a hurry to cash out. Also, as a collector, I want to keep most everything I find and certainly, many are impounded, but I have often made the hard choice to let something go with the intent to add to a current collection or take advantage of a new purchase opportunity.
The same rules of homework and diligence apply to this as they do in the stock market. It isn't for sissies, or if you need to cash out fast, as it involves waiting for the next big auction which takes months from deadline to payday.
I am sure someone here will say "buy what you love since everyone loses money with props" blah, blah, blah. It's like a mantra on many boards and maybe with a lot of the market it is true. But, there are people here and elsewhere who make their living doing it or supplement their income with such deals, whether they be vest pocket dealers or full time board sponsors. Do one thing and do it well. Know your subject and then find the pieces. Unlike stocks, you cannot order one from column A or buy on website B, but you have to be clever and lucky to find them before the dealers do. I know lots of people doing quite well with it, but they are advanced, well networked and tireless. They know, in many cases, who wants what or if there is an unknown entity active at the auctions, and then produce the pieces for sale. Sometimes you hit big when the timing is like that. I had a piece I bought in 2007 that I could not get anyone to share my enthusiasm for, until early this year I saw another sell very well and brought my to auction. This piece that I could find little interest in for a few hundred sold for mid five figures. Every dog has it's day, but, you really need to know what you are doing.