I'm hoping that SRs sell for cheap, but are still worth something (I've been keeping all of my SRs hoping to get something for them), URs will actually have a little bit of value and Epics and above are worth selling. If Epic cards can be bought for 800 credits, I don't see how that benefits anyone.
The biggest thing is that they let the market dictate value. An artificial market will kill the AH before they get it up. Any market needs to have a percentage taken from the seller - that's just how digital economies work. 5% (like FIFA) would be great for us. 15% (like 2k15) is high, but tolerable. Anything above that will hurt trade. Personally, I'm hoping for a 10% commission. I think that's reasonable for everyone.
If they do this right, even SR (or even Rare) can have value. I'd buy certain Rare, SR or UR to help complete team collections (especially if they start introducing rewards for completing collections). This could be a huge win for CD and it would add another element to the game for us.