189112 Zip Code Program

I'm looking for a Zip Code (USA Postal Code) program that I can build into any number of websites. Here's how it will need to be implemented: A seller signs up to the site to sell a product or item and includes his name, address, and zip code. The zip code will enable viewers to locate the customer's item for sale. I need the locator to be able to locate items within 10 miles, 25 miles, 50 miles, and 100 miles of the zip code and bring those items to the viewers search page.