For payment, I would think that it should be certified in some way, Money Order, Cashier's Check, PayPal (personal checks would be a mistake in my opinion)
AS far as shipping, it depends on what you are comfortable with. For some of the users that are regulars or that you know to have good reputations I wouldn't have a problem shipping before I received payment. But if you don't know them, I'd wait until the money is in your pocket.
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The clock on the wall says 3 O'Clock...last call...for Alcohol!
"I love a martini -- but two at the most. Three I’m under the table; Four, I’m under the host." Dorothy Parker