You should only buy from reputable sellers like the Perth Mint in Australia. They can provide insured courier services via DHL (insured meaning you can claim your money back if the parcel goes missing in transit) or a gold/silver ownership certificate which is guaranteed by the government of Western Australia. They are very professional. You can hand in your certificate for physical gold/silver at any time. Certificates are good because you can trade your gold/silver back to them a lot easier and you don't have to physically store the gold/silver yourself so no worries about getting stolen. They will keep your gold/silver in a vault on their premises.
Don't go looking for the stuff on the street. Most of it is smelted brass of filled gold - i.e. gold only on the outside but inside is filled with fake material.