I encourage you browse the web and see for your self, workplace psychology is an interesting study.
As for seniority rules, I don’t remember reading anything about those, but from my experience, I’m not a huge fan, I think there are better systems, particularly those which are based around consensus. However, given Starbucks’ huge corporate management, I’m guessing that seniority is miles ahead of the system they have now, which is probably management arbitrarily picking which is wide open for abuse such as retaliation. They probably picket seniority because it is very hard for management to abuse it.
Because a market requires certain conditions to be true: equal access to information, many buyers and sellers, and that the products being exchanged are comparable. It's possible that requirements 1 and maybe 3 can be met, but 2 completely kills the possibility of an efficient market in this proposal because there are far more sellers of time than buyers.
All capitalist economies (that I know of, please correct me if I'm wrong) employ regulation of market activity to compensate for imbalances in one of more of the three areas above. The theory for doing so is not based on goodwill to all people or something like that, but because distorted markets are inefficient and waste value.
To be clear, the things being exchanged here are the shifts themselves. I don't see why you can't have a healthy market between the few dozen or so people that work at a starbucks location.