What is "megalister"

A megalister or mega-lister is a large online seller of second-hand and rare books with more than 100,000 listings. Megalisters offer their books for sale at large online book marketplaces like Amazon, Abebooks and Alibris. Some megalisters may not own all the books they are selling. When such a book sells, the megalister orders it from another bookseller who really owns the book, who is offering it for a lower price. Either the bought book is then sent to the customer directly, a drop-shipping operation, or it is first sent to the megalister's warehouse and then posted to the customer.