Here are some things I do when I buy something from a seller the first time.
You should always read the feedback of the seller, especially the negative feedback. Also, carefully read the payment and shipping information. If anything is unclear, ask the seller a question. Good sellers should be able to respond promptly.
You should also check if maybe other sellers have a similar er better deal. A quick search for "SNSD star card" returned 634 results, take the time to look through them.
There is no guarantee that no problems will occur, but this should help avoid "bad" sellers.