I have been owner of a small internet retail website for about 10 years. As an "internet insider" of sorts, I have developed a set of criteria for buying from unfamiliar websites. I do a lot of internet shopping for myself and my family. Mainly I buy from sites I'd heard of. But sometimes I need to get a product from a site I'd never heard of previously. This is how I determine whether I think they are legitimate:
1. Where are they located? Do they have an address and phone number posted on their website? It's important to look for this, to determine whether your merchandise is coming from the United States or from overseas. Look for an email address, too. I am leery of websites that only use a contact form that you submit; I like to see an actual email address -- one that uses their domain, not a free domain like Yahoo or Gmail. No location, phone number, or email address listed? Big read flag!!!
2. Does the website contain Google ads on it? Seriously, only very "low rent" websites will contain Google ads. Most legitimate online retailers avoid that like the plague. Why? Well, if you're an online retailer, you don't want people seeing the URL for another website while they're on yours! Google ads are just an easy way for webmasters to make extra money with clickthroughs. Any decent internet retail site will not resort to this tactic unless they are total clueless amateurs.
3. Look at the photos and read the product descriptions. Is the English bad or broken? Are the photos blurry or unclear? Do the photos actually show the real product that you will be buying?
4. How long have they been in business? Do they have a copyright at the bottom of their website? Is their time in business stated anywhere? If not, you can do a "whois" lookup to find out more about the domain, how long it has been around, and who owns it. Just go to any search engine (Google, Yahoo, Bing, etc) and type whois followed by the name of the website.
5. In general, how does the website look? A cheap looking website does not necessarily mean a bad company (sometimes these are small businesses which cannot afford a site designer). However, a cheap looking website combined with a few of the other things I'd mentioned is a red flag, IMO.
6. Is the shopping cart secure? Look for the "https" when you get to checkout. If there is no "https" (only "http") it may not be a good idea to buy from them, because your data will not be submitted in a secure manner.
7. If you are slightly leery and unsure, go to your favorite search engine (Google, Bing, Yahoo, etc) and search the company name and/or URL. See what other people are saying about them. Do not take the advice of some of these fake "scam alert" type websites (which are often scams, themselves). Read a variety of opinions from various sources. If you have security software installed on your browser, such as AVG, that could give you a decent indication of whether there has been trouble with that website, as well.
8. When in doubt, use PayPal. If the site offers a payment option through PayPal, use that instead of just entering your credit card. It is more secure.
9. Beware of some Amazon sellers. In the "old days," Amazon sold only merchandise which it carried in its warehouse. Now anybody can become an Amazon seller (much in the same way that anybody can become an eBay seller). When you buy from Amazon, do you really know WHO you are buying from? Is it being fulfilled by Amazon or by some unknown company? Notice that Amazon doesn't provide any contact details for these third-party sellers. There are many Amazon sellers out there selling knockoff counterfeit merchandise. Remember the old saying: if it looks too good to be true, it is! Amazon is trying hard to crack down on these counterfeiters, but it isn't easy.
10. Ditto for eBay sellers. Most sellers are legitimate, but beware of counterfeit merchandise.
11. Look for small print. There is a new breed of website in town that lures you in with an incredible bargain on shoes, jewelry, or perfume, for example. What you may not notice is that along with your bargain, you are buying a $30 to $50 per month membership to the site. Before you know it, they are charging your credit card monthly, and it can be almost impossible to get them to stop. Before you checkout of ANY website, look for the small print. Pause a moment before hitting that Submit button. It might just save you a lot of aggravation.
12. If you can't use PayPal, use a credit card, not a debit card. Credit cards have more security precautions and are generally more secure. And the type of credit card you use is important, too. Companies like American Express and Capital One have excellent customer service. They will dispute a charge for you without any hassles. If I need to buy from an unfamiliar website, I usually use my American Express.
That's all I can think of for the moment. If I think of any additional tips, I'll post them.