Authentic and reproduction phonographs and gramophones are available in quantity on eBay, and this guide will assist you in determining the authentic from the repro (or imitation, as the purported repros on eBay were never actually made in the first place).
Authentic brands such as Edison, Victor, Columbia, Brunswick, and Silvertone are among the most  offered phonographs on eBay, and especially the first three. Edison phonographs are available in both cylinder and disc formats, with models such as the Edison 'Standard' selling at an average $400-$600 dollars.  You will find that many are offered with reproduction horns, however, as the original horns, being easily removable,  have become lost over the years. 
Victor phonographs are available in both the outside horn models (which have oak or mahogany cabinets with a large morning glory or black and brass horn) or the Victrola, their patented name for the phonographs with self contained horns. These come in tabletop and upright models.  Many times phongraphs are called 'Victrolas' even if they are not made by the Victor company, so popular did the name become.
Phonographs were once cylinder format only, and disc format machines were called gramophones. If you search on the word 'gramophone' on ebay, you will find a large number of machines with mahogany stained cabinets and big fancy brass horns. Most of these are reproductions or imitations. If the machine has a back bracket (this is the part that extends out from the back of the machine and supports both the tonearm and horn) that is silver in color and has a design in relief of vines and leaves, and if the horn is attached by means of a cylindrical brass tube that extends upward from the bracket and then makes a 45 degree angle forward, and if the horn slips into it without threads or a rivet and slot to hold it in place, the machine is a forgery. These machines are made in India by the score, and usually have a decal on the front that pictures the Victor logo of the dog and the gramophone. It usually says 'His Master's Voice' under the graphic. This particular decal was never used by Victor or by the English branch of Victor, known as 'The Gramophone Company'.  The cabinets are modern creations of pine or some other local wood in India, and they are stained various colors. They are constructed square, round, and hexagonal shapes, when the originals were square or rectangular only.  They contain motors swapped out of old suitcase type portable gramophones from the 1940s-60s. Many times the springs have been 'repaired' and new parts added. In most cases they do not work well at all and are not repairable. The tonearms are also from suitcase portables, and the speed control and on/off switches are newly constructed.
The good thing is that most sellers of these machines label them as reproductions, and some offer them as decorations since they do not usually function at all well. There are some, however, who are not knowledgeable about what they are and who sell them as originals. There are also a few who purposefully sell them as original intending to 'rob' whoever is taken in. Many of these dealers are in China and advertise the machines as Chinese antiquities. Beware these sellers!! They may sell the machine for as little as one cent, but the shipping cost is charged at a rate of perhaps $300 or more.
All in all it is a 'buyer beware' attitude that you must take on eBay. I hope the foregoing will help educate the public on antique phonographs and that it will assist in doing away with unscrupulous sellers.