What kind of software do I need?

[Chiana]

I know I need some software/programming to suit what I want to achieve, but I don't know what it would be called or how I find it.

My ebook website is currently set up so that customers can log in and buy members-only specials. I want to change the set up a little so that merchants can log in, place their products into a sort of 'marketplace' and then have customers buy from there.

IS this two separate sets of log ins pointing at the same database? or is this something different?

[madmax]

I think that MySQL can take care of your needs for your database. PHP along with HTML should interface it for you and all that software is free.