I'll give my own view but no mention of kettle drums you will get later from Norb.
For a shop bat I will look at the grains, shallow but true, we are all grain tarts at heart. I will then assess density, the lower the better, and if possible tap any that are still in the reckoning with a mallet. If I'm buying for myself I want performance first but they have to look good too if possible. The mallet will give you a good indication of the pressing, you want it to have a mid range sound, too high and it will be pressed too hard, too low and it will be soft and require lots of careful knocking in. After hitting bats for 20+ years I leave it to instinct and it is hard to convey the exact sound, John Newbery would tap all his bats and proclaim it's a "Screaming Cat" to Tim and Julian when he got one, I'm sure that sound will still bring a smile to their faces.
I tend to avoid poorly made bats, seen some lovely clefts butchered on the press and also ruined by poorly fitted and shaped handles. I have had some of my best ever bats from James Laver but still cannot stand his handle shape and have then rehandled now before use.
As before I think that low density plays a very big part in potential power, it needs to be pressed well and those 2 factors are the biggest, super stiff tight grains are too hard to find to go looking for too much.
When I have bats made the situation is simple, I have my own willow, I know most places here and abroad that make and can pick and choose, I choose the best possible quality and price plays no part.