My "walking system" is simple: for any action, I secretly pick a number between 1 and 20. You state a number, I compare, and then decide what happens based on how far off you were. Initiative and damage the same way, but using 1 - 10.
So, as GM, everyone has to trust you. I'd suggest you simply tell everyone when they post their activities that any action they think they should roll for is posted like so: I pick his pocket . Whether they select a number or roll a die for it is immaterial, since you're comparing their number to yours, and they can't cheat by trying to pick high or low to fit your system.
When you respond, you put it like this : "You tried to pick his pocket [18 / 3]. He easily detected your clumsy attempt to snatch his purse, and now he's trying to punch you. Initiative call."