They are all brandable. They all start off at $0-$8 value. It's the perception of the profitability of a brand which makes one name more valuable than another. If one business is or is projected to make $1M/year, it has greater value than another business which doesn't even get off the ground. Since more businesses fail than succeed, most of the brandables you see on the marketplaces are way over-priced. I don't really know the success of sites like brandbucket. But if we say conservatively they turnover 10% of their stock every year. That means between 90% don't sell in any one year. After 1 year another 10% of the new listings sell, and the old 90% from last year, 80% (or more) remain unsold.
Now. Some people can make a success at trading brandable domains. But most will fail. Take a look at those brandables mentioned above that dropped. They're all crap. Which is why they all dropped. Any businesses that used them all failed. It would take a profitable business to make any of them valuable.
So we come to what makes a crap or good brandable. If all those domains which dropped are crap. What is the common theme running through them. They are all 5 chars. Short is a good quality. But none of them are evocative. OK. Branding can take care of some of that. But evocative in itself is much easier to brand than unevocative domains. None of them particularly roll off the tongue. The more easy to pronounce a brandable domain is (the telephone test), the easier it will be to sell, and the more likely it will succeed. Leafy.com (ok a real word) would be much easier to brand than any of those dropped domains.
Just my 2 cents worth of opinion.