MONEX (8.5/10 Stars) http://www.monex.com/
Advantages: The largest North American bullion dealer. They do 6.5 billion in transactions yearly. Prices are posted on their website and they usually are the lowest priced around. They sell Silver, Gold, Platinum, and Pallidium coins and bars through all the major mints. They offer delivery, 3rd party storage (through Delaware Depository), and trading/leveraged accounts up to a 75% leveraged position. Great referral program offering 3oz to 10oz silver for each referral. Fast delivery times. Can order 7 days a week.
Disadvatages: They have minimum orders depending on the product. No online buying. They don't take credit cards (but do allow you to place order and send in payment within 24 hrs). Their leveraged program can produce fantastic returns but it isnt for everybody.
KITCO (8.5/10 Stars) http://www.kitco.com/
Advantages: Very informative company web page with great news items. If you are selling they purchase bullion and jewerly products with fair market prices. Good forums. Can buy smaller quantities starting at only 1 coin. 10oz gold bars are the best priced of all the major companies. Online ordering with credit cards.
Disadvantages: Prices tend to be somewhat higher unless you purchase $100k+. A lot of the more popular items have longer shipping times.
GOLDLINE (7/10 Stars) http://www.goldline.com/
Advantages: 2nd biggest bullion dealer by volume. The offer bullion as well as numismatic products. They've been around for 20 years.
Disadvantages: They don't have transperant pricing online. Most of the time they will try to upsell you into higher premium exotic coins, Money has to be in-house before you are able to order. 6-8 week shipping time.
APMEX (8/10 Stars) http://www.apmex.com/
Advantages: Online ordering with a good product line. Can pay by credit cards. No minimum quantities for ordering which is great to use if you are ordering only a few coins at a time. Online ordering 365 days a year. Numismatic coins.
Disadvantages: Higher prices on most items compared to their competitors. Some products are out of stock. Longer shipping times.