Hi Serena. Welcome.
If I knew of three places where I could just as easily get the same product and I knew that the only meaningful difference was the price, there's no way I'd make my purchase from anything but the cheapest one.
Oh, yes qwerty, that is in the back of my mind as well - and a major problem. Don't think I haven't considered it - I actually worry about it quite a bit.
It is a blessing to have three stores all with the same products because they all rank differently for this, that and the other thing but the good part is being able to spread your content out across the net in different arenas. What doesn't always work for store1.com may work for store2.com and so on.
I guess the idea is scrapped - just need the the final validation that it won't really work after all. The better solution in my mind is to work on producing content and give all this ranking madness a rest for a while.