Ecommerce Website

I need to have a ecommerce website similer to this www.instantbooks.info. All features required such as instant delivery of ebooks, paypal integration etc. I need a backend interface where I can add/remove the products. I also need a feature where users can register to sell their own products & I can control the sellers from backend. Also, I need to have an option for payment gateway integration with other payment gateway's like ccavenue & paysignet.com. I don't want to use them now but I might want them somewhere in future. So, If you can leave an option for that. Your suggestions are welcome. PS: If this project is successfull I may want to use your services for ongoing basis.