[downandout]

After the 20% they pay to Uber, and income taxes on the money earned from Uber, they would wind up with half of the miles they had earned through this "money" layer. With this, they get full value for each mile. Additionally, Uber is restricted from entering into many markets (example: their failed entry into Nevada, where one of the largest tourist markets on earth is located). With free - as in no money changing hands - rides, presumably the app would be allowed everywhere.

[pdabbadabba]

You don't need money to change hands for a transaction to be taxable. The sort of barter-based system you describe would also (in theory) be taxed. The only difference is that tax enforcement would be much more difficult under your system.

That would work well for a while, but if your service got really big, and became a target for enforcement, you (or your users) might have serious tax problems.