Wow, I'm genuinely amazed by the number of negative replies about online retailing in 2011. I use online shopping for just about everything from my groceries to clothes and office supplies.
There are various symbols you can look out for to verify a site is genuine or not, but there are so many well-known online retailers that safety is rarely an issue.
The main thing is to remember that if something looks to good to be true, it probably is. If a person on eBay who you've never heard of - and has no reviews - is selling something for £500 cheaper on eBay than even the largest company is selling it for, the chances are it's a scam. As for contacting the supplier, I'd imagine someone running a scam would be more than happy to reply so I don't think that would necessarily work.
Reviews on an independent and reputable review website that go back over a number of years are a great way to check out a company's history, as there would be no shortage of bad reviews if they were cheating their customers.