I am not sure about USPS, do they have an automatic insurance ammount attached to a package? Canada Post has $100 insurance automaticly, if your item is worth more than that automatic insurance I would also HIGHLY recomend purchasing insurance to cover the full cost of the item plus a bit...this way if anything happens you and the buyer are covered.
Paypal is probably the best and safest way to both make and accept payments. easy, quick and it has built in anti fraud and dispute functions.
Generaly accepted practice is the buyer pays you and you ship. Trades are a little different.
Always check the feedback of anyone you are doing business.
Flat rate boxes are your friend for anything heavy. Small light items can be shipped cheaper through other means.
If your shipping outside of the US, ONLY use USPS!!