Its going to be very hard to make this a win.
Trying to compete with template monster is ridiculos, obviously you want to aim high but imo you cant touch them. They are a multi thousand dollar company..
Also, the idea sounds good but as robson said you need to populate the site ith high quality templates or basically no-ones going to buy them..
Why will people want to sell templates through ypur site if they can do it for free at WHt or Sitepoint? You will need some major advertising to bring in the numbers of p[eople needed to make this big. be prepared to dig deep into your pocket with this venture, and as always good luck