The good thing about the boom market in precious metals and especially silver for investors this time around is that with the internet, everything is so much more transparent.
Although, the term "caveat emptor" or "buyer beware" will never go out of fashion, nonetheless the playing field has been levelled considerably between new and experienced investors.
And for me the quick answer to identifying a reputable seller, a fair price and genuine silver bullion product is very simple. I go to what is now the biggest market there is - Ebay - http://cgi.ebay.co.uk/
Ebay is the ultimate marketplace as the market will dictate a "fair price" by accepting bids from interested parties, you can view the sellers credentials especially the feedback that they have received from other buyers to decide if they are a legitimate seller, and if you have any problem with the quality or description of what you buy, you can usually get your money back, through the Ebay/ paypal chargeback procedure.
If you're interested in acquiring good value, certifed silver from a reputable and honest seller, then why not visit this auction - http://cgi.ebay.co.uk/