Hi Victoria,
Doing it on your own will create you a lot of problems if you don't know anything about computers.
I suggest you try these two known services:
- Ebay:
eBay - New & used electronics, cars, apparel, collectibles, sporting goods & more at low prices
- Amazon:
Amazon.com: Online Shopping for Electronics, Apparel, Computers, Books, DVDs & more
Both have all the tools needed to sell anything. From selling individual products to setting up entire portals with them if that's what you need. Amazon even lets you outsource with them your entire warehouse/shipping infrastructure. Both provide everything to get you started, and you don't have to know anything about computers. Most of the questions you asked are automatically solved with them.
Another very important thing they'll provide you with is TRAFFIC!! You'll need it if you want to sell something on the Net.
Maybe eBay is more adequate for you business, but you'll have to judge it by yourself.
Good luck!