The most commonly seen reproductions are the ones from American Oil, which were gas station giveaways back in 1972. They are easily identified by AO-1972 found on the curl. They also have the candidate's name and date of the campaign on the curl. Sellers on eBay often show a scan of the latter and not the former. Watch out for these sellers because this is an attempt at deception. Of course, legitimate button do not have the name and date on the curl.
On any given day there are a multitude of these worthless pieces of tin on eBay. They are often sold in complete sets, but authentic buttons are not found in organized groups representing one button from each campaign. There is an obvious lapse in logic here. Many sellers claim they did not realize they are fake. I must be blunt here, but only someone lacking in brain cells would think one of these sets is the real deal. For one thing they are all identical in manufacture, despite the fact that they span campaigns from 1896 to 1968. I can't think of anything man made that hasn't changed in some way in 72 years. Also the identical coding system and numbering should make it obvious to anyone.
The buttons in the American Oil set are lithos, a manufacturing type that was not in use prior to the 1920s. Lithos are actually easy to determine from scans. Examine the look of lithos vs. cellos, and after awhile they will be easy to spot. Also look for scratches, which is a dead giveaway for lithos. If in doubt just ask the seller. Of course if you see a McKinley button with a minimum bid of .99 and no bids, chances are it is a fake. Do not buy them under any circumstances. They are completely worthless. Reproductions are excluded from price guides, and they are banned from all political button shows.