Do different businesses sell stock at different rates? If they are the same, what is the inventory rate sold per hour?
That is a complex question. It seems that the rate of sales is fairly constant from level to level. But, that rate is determined by which elevator you have. The elevator determines passengers per minute, which in turn, determines sales.
The floors hold different amounts of stock, and therefore sell off at different times even if the rate is similar.
Other factors determine sales rates: for instance buying frenzies, VIPs such as the celebrity, or big spender, etc.
I do not believe that rank, or skill have any determination about sales rate.
After doing some testing, it is apparent that the elevator rating, PPM, is how many customers come to each level each minute when the game is turned off. About half that rate come to each level when you are actively playing the game.
If the level has stock, it is purchased evenly. If you've stocked all 3 items, and you get 6PPM, then on average each minute each item will sell twice.
This means that if you stock only the top 3 credit item you'll sell 6 of them per minute, which is more profitable than having everything stocked.
There are other temporary effects that will change a given floor's PPM, such as a celebrity, but generally the PPM rate is customers per stocked floor, per minute, when the game is off.
The game is still being adjusted over time, this is as of the 1.3 update in early February 2014.