Custom PHP modules for an auction website (MODx Framework)

This website will offer hotelrooms/trips/airfares/etc on auction from overstocked inventory of hotels, travel agencies, etc (called advertisers below). Visitors to the site can register as customers and bid on the products on offer. There will be a large turnover of products and visitors. We will build the website in MODx (design, templates, structure). We are looking for PHP-programmers who can provide us with the various custom modules and/or snippets of code that relate to the auctioning, customer and supplier registration, invoicing, backend management. What we need our supplier to deliver is the following functionality (see attached pdf for flow chart): Two types of registered users: customers (use standard MODx webuser module) and advertisers (program custom MODx manage module (PHP)). Customers will be web users. Advertisers will login tot the MODx admin panel and manage their products and account info via a custom MODx module. - customer registration - customer bid - customer winning bid --> payment gateway - customer payment rejected return to winning bid - customer payment successful on to voucher - customer receives voucher - customer cashes voucher - Advertiser registration - Advertiser adds product (holiday/hotel/airfare/leisure service etc) - Advertiser accepts terms/contract/auction fee - Monthly invoicing to advertisers The application must: - Be SEO optimized (semantic css/xhtml/friendly urls) - Have a backend that allows maintenance by a content manager - Fast rendering of the pages - Flexibility in managing the products (timeplanning, amounts, pricing, specs, visibility on the site) - Prepared for multilingual usage Examples of similar sites: [login to view URL] [login to view URL] [login to view URL] (different kind of bidding, but otherwise good example).