I donno but I think this is Question one:
Does something like this exist already and are people actually paying to use it? (Unless of course you're selling a (great looking) service for all the other questions).
If it's really "killer" (not as in: "I'm creating a social app where people hunting cows with bows can use their smarthpone to share videos of arrows piercing bows in real-time") then people may not know they'll actually want this.
I don't disagree that having other buying apps using your idea is good in that it shows there's a market.
But then you don't "Have a great idea". Because you're idea simply is "My idea is to copy another product".
Not that I have anything against it: it's what I do for a living : )