Now, the items that give advantages need to be small advantages and not to the point where it's totally gamebreaking... You may even get away with just selling high end items that can be earned in game as well. But I'm positive that powerful items that can't be obtained in game will sell the best.
Yes, you will have a vocal minority that complains *A LOT* but the end result will be a huge increase in profit. People have misunderstandings when it comes to what "free to play with cash shop" actually means. It means "if you want to play this game seriously or on any sort of competitive level, you're going to have to pay to get there". For most people who just want to play for free, they are fine with knowing that they are at a slight disadvantage to those who pay. You get what you pay for. Too often people just assume "free to play" means "I should never have to pay for this game and should be able to get everything I want/need without paying".
To further strenghten this, I would point you to Astrum Nival, GPotato and the game Allods Online. This is the best example of an awesome game being destroyed by a cash shop tweak. And this damage cannot be undone, no matter how much you try. I admit this situation is specific, as the game was oficially still in beta while cash shop became very expensive -- the producers must've hit themselves hard on the head to pull off something like that .