Before you buy, the most important thing is to look, look, look. The more time you spend studying old cards the better you will get at spotting genuine antiques. Only buy cards that have a legible postmark on them. If a card is postmarked 1914, for example, you can be reasonably sure the card is at least 86 years old. Notice the thickness of the card stock.
Modern cards tend to be thinner and feel glossier than antique cards.
Notice the overall condition. Check the design of the card. If the paper feels right and the back is undivided, it probably dates before 1907. Read the small print. As you begin to amass a collection of postcards you will want to store them.