Simple PHP Best Offer Script

We need a script developing that works similar to eBay's "Best Offer" feature. This script must be very very simple as we only have a basic website. We want to list products on our "Special Offers" page that have minor defects. There will be up to 4 products. We want customers to be able to make an offer for a product i.e. £200 and we will set a minimum price for each product i.e. £250. If the customer's offer falls BELOW our minimum price, the system will say "Sorry, your offer has fallen below our valuation. Please make another offer". If the bid meets or is above the offer, the system will show "Your bid has been accepted. Please proceed to buy now" at which point a simple PayPal button will be displayed that takes them to a simple PayPal checkout. I'm sure something like this already exists, we can't find a script to use. We need this quite quickly and are sure it is a simple job for a proficient coder. Please bid your turnaround time and availability.