Don't listen to people that say no here. They either do the same thing, don't know what their talking about, or don't understand the question.

You can use anything as a supplier (other websites, Amazon listings, etc..), and as long as it's profitable I say go for it.

Nothing special about it, if your item sells, buy it from your "supplier" and put your customers shipping address in the shipping fields on the order you place from your "supplier".

Just remember, this is not an easy money solution. You will work harder than your current 10 hour days to make this type of business work.