it 'only' needs a database design with some more tables to accomodate a multi-seller application. And, of course, a somewhat extented GUI to input and output the additional data.

So what's your exact problem?

Any book/course on database design, webservers, database servers (like SQL Server or MySQL) and server-side languages (like .Net, or Java, or PHP, or some combination) might be a start.
However, first read Sovereign advice in post #2 of http://forums.cnet.com/5208-6616_102-0.html?forumID=37&threadID=257503&messageID=2546486

My students might be able to make this after a 4 year IT study. Maybe better have some company do it for you.

Hope this helps.


Kees