For me it's only pay to win if the performance-changing items you buy with real money can't be gained through normal gameplay i.e. Sword of Awesomeness with +99k to all stats is only available for real life currency.

Going with your example of people selling credd for gold, to me that's not a whole big deal. All it does is just speed up the process of you acquiring those items. It's a slight advantage because those items are available to everyone else in the game. Eventually people will own those same items without paying a single cent and that advantage loses it's value.

Personally, i think some people can benefit from the credd system. In my last mmo game (swtor) our main tank couldn't play for more than 3 hours a night due to RL. He did not have the time to farm his dailies or commit to crafting so he was always broke-ass. He wanted to pay real money for gold so he could afford repairs, consumables etc, and there was no ingame support for it. So he had to buy items through the cartel shop and sell it for gold via auction house. All of this is legal via in-game interface. He did this so he could play with us and we too didn't want anyone else to tank for us. In a sense, you could say the credd system was already in place - just not as direct and obvious.

I'm waiting for dev to start pushing out the ultimate model. Win to Play model.