Those are questions that do not work across the board with product.
It is a complex topic, but I will try to simplify it.
Essentially, for any product, you want to be able to analyze:
1. The price you pay against what it will sell for. A healthy ratio is to double your money per what you paid.. Pay $50 for it and get $100 at sale..
2. The rate in which the product sells a day on eBay.. Like 4 a day, 1 a day, 1 every couple days, etc.
On top of the above, you have to know how to sell, etc.. My website helps in this area..
You get better at it as time goes by. You WILL make mistakes, but you will learn nothing if you do not make mistakes.. Business is not like school. In school you are told it is wrong to make mistakes. In Business, mistakes are a tool for learning.
You don't have to sell CDs.. You can sell just about anything..