Visa Cards - I may be buying a couple if you tell me the basics

Alright, in the past week I've become very interested in Visa Prepaid Gift Cards. I have a few questions, though and I'm hoping someone will be kind enough to answer them. Italics are the most important ones.
If I buy a card from someone in the US, can I use it in Australian online stores?
Is it illegal to buy/sell visas?
Will online companies accept them and ship to whatever adress you provide?
What verification do online companies need?
Do you have to be 18 to buy/use one?
What's the ratio for buying off here i.e if the card has $20 on it, how much will I pay on average for it?
Do I need the physical card to buy stuff in REAL stores

Thanks everyone, any help is obviously appreciated.
Matt.

 

You can use it anywhere, just like a regular visa, I know this because when I went to Germany, I could use my visa, the only thing is, exchange rates, they might get you with that.
Online Companies should accept it, I don't see why not, as long as you enter what's on the card, you're fine.
I'm not sure what you mean by verification? Normally it's the name on the card, expiration date, and card number, and SOMETIMES the security code on the back.
No you do not have to be 18
Also, I missed one, I do NOT believe it is illegal, but you might want to google it just in case.
Not sure of the ratio.
AND YES, you WILL need the physical card to purchase things in store, because when you use a credit card you have to sign for it, and it's a security measure that the card is PHYSICALLY SWIPED, the company I work for has had MAJOR problems with cashiers keying in cards versus swiping, because you can easy claim fraud on non-swiped cards versus swiped cards.


Hope I helped!

 

Anyone else? Does anyone have any they would be willing to sell?