BarnesandNoble.com, Amazon.com and ThriftBooks.com all have extensive used large-print selections. The best way to meet your needs as a customer is to compare the specifics each site conveniently includes in their product pages for any used large-print book you would like to purchase.
After identifying the title you'd like to purchase, locate it on each bookseller's website. Both Amazon and Barnes & Noble promote secondary sellers to market their used books through their websites. This means that there may be many available versions of the same book. When you click on a link directing you to the available used copies, the websites then direct you to a list of these secondary sellers and their inventory. Here you can find extensive ratings of the seller's past experience with customers, as well as an indication of the quality of the book you're looking for. ThriftBooks operates a bit differently, as it is a primary bookseller. Rather than pay Barnes & Noble or Amazon to market their used books, they choose to sell on their own. This means they may have a less extensive stockpile of the same title, though it also means you'll be directly dealing with them, rather than through a larger corporate bookseller.