RMAH doesn't makes the game "pay-to-win" as long as it stays in PvE. However, PvP is all about playing against other players, and unlike the AI controlled mobs, players can get frustrated, pretty eaisly especially in Diablo, because if they'll be disappointed in PvP, nothing can save this game (from their perspective). RMAH should not effect PvP.
But how? Somebody buys something good in RMAH and he can't use it in PvP, but he can in PvE? That's stupid, now the RMAH user becames annoyed, feeling he is betrayed by the system he paid for.
The only solution to make PvP fights balanced and skill based (as it should be, if you don't agree with that, you just want to win, no matter if it was your doing, your skills, or just your wallet and a bad class / skill / item balance), is to make PvP gear and the chance to get better PvP gear equal to everyone.
Check Guild Wars 2 - it has the most balanced PvP in online game's history.
Everyone starts with the same level - even if you are lv5, when you enter PvP mode, you get to max level (only for PvP of course), you get all your skills and everything unlocked. Diablo III has no problem with that.
You lose all your gear, but you get much much more stronger ones with godlike stats from the very beginning. Everyone gets those. Damage scale can be controlled this way, so noone get's 1 hitted because bad item balance, and skills are easier to balance (from Blizzard's perspective) than items, because those effect all classes and all players, including the economy.
There are NPCs you can "buy" PvP gear from, for totally free - it's equally strong compared to the good one you get at start, but made for different builds. Totally should be in Diablo III.
You get points in PvP for scoring (capture), kills, helping (heal, buffs) others, and you can spend these points at the mentioned NPC for a higher tier gear. How I imagine this in Diablo III: you level up your "PvP level", and when you reach a point (let's say PvPlv10) you unlock the next tier. You can buy the new items, and once you bought a piece, that will be free in the future but only for that character.
So, what we got?
Skill based fights, it matters if you are smart, got good reflexes.
Easier balance for Blizzard: all they have to care about is PvP gear (equal for everyone in the same class) and skill balance. The economy can't be fixed, deal with it, and if it poisons PvP, Diablo III is totally dead.
Ladders and scores will show how good you are, instead of how rich you are.