Main advice would be to pick a known brand and buy from either a bricks and mortar store or a site where you know who you are buying from and are certain of the returns policy.
eBay and similar are to be avoided at all costs. There is a common scam where a small card is doctored to look like a big one, sold at a "great value" price, and if you are lucky all that happens is that by the time you realise they have closed up and opened another eBay store instead, leaving you out of pocket. If you are unlucky you don't realise until you have lost important photos or similar because the OS thinks the card is bigger than it really is. And this is so common that all I can advise is not to take the chance.
