Betts Bookstore This is THE bookstore in the world of Stephen King. Exclusivly online they have almost every Stephen King book available for Sale
Veryfinebooks.com An established SK retailer and very friendly owner. All books come with a 30 day money back guarantee. Also will sell your book on consignment. You name your sale price and the fee is a very reasonable 15%
Gerald Winters & Son - Recently opened in Bangor Maine by longtime collector Gerald Winters this new store has a lot of rare King books as well as rare works by other authors.
Camelot Books I've purchased multiple times from them and have been very satisfied; they answer e-mails fast and friendly AND items have always arrive gift wrapped.
Badmoon Books Roy's place is on the opposite shores of the US (in California) which is great for people living on the West Coast. He often gets limited amounts of S/L books for pre-order.
Hutch's Rare Books A long time buyer and seller of rare Stephen King books.
Broadart.com This is the company that makes archival quality mylar covers to protect your dustjackets, as well as many other library supplies. Order direct from broadart and get exactly what you want.