Recent events have encouraged me to write this thread; I wanted to buy a game off of Google play, there didn't seem to be any problems but it didn't appear on my credit card statement.
Last week 73 transaction from my card to everywhere around the world... And getting all the money back is, trust me, a pain in the ass.
My advice is to always use PayPal, and buy only from sites that offer PayPal payments. Anything that directly asks for you card details is a potential scam. If they can't get PayPal (or Moneybookers, another major internet payment service) they're most likely not legitimate. Another safety measure is to have a separate account for online purchases; so you can add the exact amount of money that you need to that card, and have it empty when you're not purchasing anything. That way even if someone gets to it, they wont manage to steal anything from you.