JM Bullion, Provident, Modern Coin Mart, Liberty Coin & Precious Metals, Silvertowne, Gainesville Coin, Goldmart and Monarch Precious Metals are good.
I've personally dealt with: JMB, Provident, MCM, Liberty, Gainesville, Goldmart and Silvertowne. The only one I can't vouch for is Monarch but I have heard they are quite good as well.
BUYING: 2001 UD NFL Legends Autographs - Daunte Culpepper - $300. Steve Young - $350.
BUYING: 2004 Ultimate Collection Jsy/Autos/25 - David Carr, Tom Brady - Paying $550, in case you like money.