Building a niche auction site

Ignore the budget, I have no idea what something like this will cost. I am wanting to have an auction site. I am not sure whether or not to add it to my existing site or have it be stand alone. I will need help to develop the concept and finally install and make sure everything works. I want the site to be niche but have the ability to expand if it works. I want it to be like a mini ebay. It's only for collectibles. I want to be able to charge sellers fees and have most of the luxury that ebay has. I don't know if there is scripts available. We can use whatever DB is needed, and space is no issue. The existing site is based on a joomla 1.0.12 core. Or we can put it on it's own domain and just link from the main site. Please pitch your ideas and price to work on this. I'm not looking to go broke on building this but if the price and project is reasonable I will move forward with the project. Please ask whatever questions for this, I have never done an auction type site, so I am not familiar with everything I would need. Please recommend what it should be built from (i.e. php, asp, jsp...etc)