Build a Website

I’m looking into making an eBay listing service. It will be in at least 3 formats. Website (Similar to Auctiva and InkFrog) Desktop application that can be ran offline and will work much faster. Possibly a third option would be a mobile app similar to the website. The new listing service website will need to follow all eBay regulations and rules including all of the latest requirements from eBay in their last update. This website will always need to be updated and managed (possibly by the company that will create it. This will be a subscription service site. It should be able to handle upwards of 20,000 users. It will allow limited image hosting and unlimited listings to ebay.(depending on subscription level). It should allow people to sign up for 30 days for free with no PayPal payment or CC info required. We need customers to sign up and get used to the service and get a bunch of listings active and committed to the service. We will not charge them for using the service unless they make money on eBay each month. (maybe a minimum of 5 sales/transactions a month). It will need a front end system that the users/customers experience and also a management side with all control over the system. (Financial, help desk, running reports on any and everything you can imagine, bulk emails, announcement splash pages when signed in at times, ect). The website version will need to be able to create listing from scratch completely and then post to eBay at the posters request. (save and close and finish later, Post immediately, schedule for later, ect.) It will need all fields that eBay allows. The service will need to be a one page lister. There could possibly be a pop up page when doing the item specifics page. I understand that the item specific page will be dynamic and will change when the categories will change. As the users create listings, it will build a library of saved listings. This will allow users to just open an old listing and modify it instead of always creating listings from scratch. Users should be able to create custom templates and or select pre-made templates or layouts. It should be very simple. There should also be a bulk editing of saved listings, active listings. You should be able to change anything about this listings. There should be a grid view kind of like excell where you can create listings in bulk instead of one by one. I need an option to add a field to add a purchase price of inventory. The site should track all expenses and give a detailed profit/loss report. (ebay fees, shipping, packing, COG, ect) The site should be able to leave feedback automatically. Email invoices and reminders. I understand this will be a big project and there will be a large design phase/layout phase before any real coding can continue. Please place a bid on how much you think your company would charge and a time frame. I have the budget set from 3,000 - 5,000 but I understand if it could be more.