Buy a Sneaker you are familiar with
Different brands and types of footwear are not always going to fit you the same exact way. For example, one brand of shoes may be more narrow than another. How can you tell if you're buying the right size for you in a particular brand? Don't guess. Stick with a brand and size you have bought in the past. Why take a chance on buying an uncomfortable pair of shoes?
If you are not positive of your size in a specific brand, I would recommend you visit a sneaker or shoe store in person. Try on a few pair of nike or reebook sneakers at a sneaker store. It is cheaper to spend a couple of bucks on gas and drive to a store as opposed to dealing lost shipping cost and the disappointment of purchasing a shoe that might be uncomfortable.
Stick with name brands
Certain brands like Nike, Asics, Converse, Adidas and New Balance are staples in most of the athetic stores we frequent for a reason. They are made well. Buying a cheap, no name brand online is probably a bad idea. If you are looking for a cheap sneaker, go to a big box store and see them up close. It will be easier to return them if you don't like them right away. I'm not advising that you overspend on sneakers, but there are plenty of fair to aggressive deals on the name brand sneakers above.
Authentic sneakers only
It can be very difficult to identify a counterfeit sneaker. Fakes are most common with the most popular and expensive sneakers series (makes sense). For example, you are more likely to find a fake Air Jordan than a fake womens running shoe. Fakes are a problem for buyers and sellers alike and most people either aren't savvy or bored enough to research a pair of sneakers for authenticity.
The biggest problem with fakes for consumers is that some are made of lesser quality materials. If you bought a fake sneaker that was made with the same materials as the original, the odds are you would never know. Buy a pair of sneakers that aren't made with the same materials as the original and you will probably be disappointed. Cheaper materials could mean that a sneaker isn't as comfortable, supportive, or durable as the original. If you do identifiy a pair of sneakers that you bought as less than original, the seller will probably refund or work with you in some capacity to improve your purchasing experience with them. Fake sneakers are unfortunately just part of the sneakers industry and this probably change very much any time soon.
Buy from reputable Sneaker stores or sellers
If it's a brand new sneaker, it might only be available at stores like footlocker and sports authority. It may even be available directly throught Nike, Adidas, or whomever. A lot of independent dealers don't stock sneakers when they are new because they only buy recently liquidated and overstocked items. A brand new sneaker won't be available to them yet. You will spend top dollar buying a sneakers this way, but you will get what you are looking for.
If the sneakers you are considering have been around for a while, then they are probably on ebay and as some of the smaller shops. The risk should be a lot less because the price will be considerable lower than the sneaker cost new at this point in time. Before you buy the sneaker, you could contact a seller with a question or two. A quick response back to you probably means you have an attentive seller. A competent seller or store wants you to keep coming back to buy more inventory from them and in most cases will do the right thing if you ended up with a product you were not happy with.
Do some research
When looking for a specific sneaker for an activitiy like running or tennis you should do may benefit from a little bit of research.
Scenario: I want to start playing tennis again. How do I find out Which tennis sneaker or sneakers are the best? Visit some sites that do are specific to your activity but do not have sneaker sponsors. Look for a site that is not profiting from sneaker sales. You are more likely to find some honest and helpful advice. This kind of information can exit in forums too. It should only take a few moments to do some research on yahoo or google for this type of information.
If you are looking for an expensive retro style sneaker, it would be helpful to check out some sneakers forums. This can provide you with price range information and what to look for as far as authenticating a shoe that could be an expensive fugazy.
Sneaker shopping closing comments
Following the buying tips above will allow you to focus on the features and price of a pair of sneakers you may want to purchase. I've bought stuff online that ending up being less than what I expected and it can really stink. Most of us do not have money to burn so it pays to be a little cautious and savvy when buying anything online, even sneakers.