1. Depends on what you are aiming for. For something like ebay you would probably want one of the mainstream commercial databases. If it's to be fairly low volume then you could probably get away with something cheaper like mysql or free like sql server express.
2. You mean do you need structures for holding accounting info for customers and the site (sales ledger, general ledger) - I would have thought so.
3. Lots but you probably don't need them. Pen and paper works well.
4. No idea what this means.
========================================== Cursors are useful if you don't know sql. DTS can be used in a similar way. Beer is not cold and it isn't fizzy.