Silk Road is building a market place via tor, on an eBay kind of model.
Yes. Though it's only like eBay in that sellers receive feedback.
Apart from "marketplace", what's similar about your service? Again, have you been to Silk Road? The types of things that are sold there aren't exactly conducive to a Groupon type thing. I doubt "much higher exposure" is on the list of goals for SR sellers.
And on top of that, you want to convince businesses to accept BTC with it, so you
can't be similar to SR. I'm sure a business would
love to run a "deal" on such a site.

I'm not saying that "Groupon with bitcoins" is necessarily a terrible idea. I'm saying it's completely unrelated to SR and advertising it here is basically spam.
Also I'm not sure what "a lot of bitcoin owners but very few bitcoin sellers" means. People selling bitcoins? People selling things for bitcoins? There aren't as many people taking BTC as payment as we'd like, but the way to change that isn't "Hey, offer a discount for paying in this currency that you don't even know exists."