You can buy coins and sell the stuff for pokedollars.
Being able to buy pokedollars for infinite amounts for real money would destroy the server's economy and cause massive inflation.
Pokecoins work because it's just the same ingame money being traded around, but if you give the players a way to buy infinite pokedollars the prices of everything would go up in ridiculous fashion.
Sounds like a harmless idea at first but i don't think it's viable =/. That said, we do need more ways to earn pokedollars ingame.