Take gold eagles: Typically the main dealer that buys large quantities from the US Mint charges their sub dealers a 3% premium ( the premium gets larger for half, quarter and one tenth ounce coins). The sub dealer can then charge the consumer a 7% +/- premium. When it comes time to sell most dealers will give you a bid based on spot prices i.e. no premium. So there is roughly a 7% spread between bids and offers. Then, assuming you pay taxes, there is a 25% or so tax on what you made. This means that if you are right about the market you have a potential 30% "attrition" factor. Checking this against eBay: Recently one ounce eagles have been trading for around a $ 100 per ounce premium over spot. So... say you sell today at $ 100 premium you then have to factor in eBay commissions, paypal charges and shipping PLUS some counter party risk PLUS our friends at the IRS - and don't think they won't check your eBay activity if they think you are a serial non tax payer. All these costs add up. Roughly the same formula applies to silver eagles. One hundred ounce silver bars are also popular but at some point, if their value increases enough, people will start requiring a new assay. The bars also weigh around 7.5 pounds each. Try lugging a few of those around!
So.. yes bars and coins and bullion you have in hand is a good insurance but do not do this if you are trying to trade as the odds are against you big time.