Android and iPhone mobile phone development

I want an iPhone / Android application for my website. The website is an e-book distributor where members can make an offer on the RRP price of an e-book. Offers can be accepted or rejected by the seller, either manually (they are notified of the offer and can accept or reject it). or automatically (they set a minimum price of which they are willing to accept). If accepted the buyer can purchase the product (via Paypal) at the price that they have offered.

The mobile app needs the following:-

1.) ability to register to the website
2.) ability to login and browse products
3.) ability to make offers on products (as a buyer)
4.) ability to accept / reject offers (as a seller)
5.) ability to update status / accept friend requests / send private messages
6.) have a built in e-reader
7.) have social media integration (tweet / social bookmarks / facebook likes)
8.) tweet an excerpt from a book to twitter etc

Fixed fee bids only.