If you an idea or expertise to do something like this I don't know why you need to wait for Y Combinator to pick you up before you get started.

Making something profitable from day 1 is difficult but profitable from day 30 is less so. Yes the profit may just about buy your team a round of coffee's but at least you got off your mark. Your ball is rolling.

Waiting for seed funding before building your product seems somewhat counter-intuitive to me.