Ive only had one bad experience with buying online, i guess im a lucky sob (/me touch wood). A seller on Ebay wanted me to pay 40$ and asked for government papers for shipping. It was for a 10$ small piece of aluminum barrel for a 1/35 model kit , that's my only negative over 180 auctions ive won. Ive learned to ask for shipping cost before bidding.
But there are some other things to check before buying here like it was said.
Traders rating, age verified, personnal infos are one of them. If you are still not sure find someone who can vouch for the buyer/seller here on ASC. But even with those infos, shit happens sometimes.