A facsimile jacket is a modern reproduction of a book's original dust jacket, made to dress a copy that has lost its own. They are sold openly by specialist firms and are legitimate when described honestly, but they are not original and add little to a book's value.
How to spot it
Facsimiles are usually printed on modern, bright white, smooth or glossy paper that feels different from period stock. The colours can be slightly off, the image a little soft from scanning, and the edges cleanly cut. Many carry a small note such as "facsimile" on a flap or the inside. Original jackets of the period usually show some age: toning, fading and wear that matches the book.
What it does to value
A book in a facsimile jacket is valued essentially as a copy without its jacket, with the facsimile adding only appearance. A dealer must state that a jacket is a facsimile; one sold as original without saying so is misdescribed. Facsimiles are useful for display and to protect a jacketless copy, as long as everyone knows what they are.
Read facsimile dust jackets to learn the tells in detail, and dust jackets explained for what an original should look like.