Reproduction bayonets are all over the place, being imported from China, Pakistan, India & other countries.
Know what you're bidding on.. Examine the photographs closely.  Pay attention to markings & all the details, ask questions and ask for more photos if needed.  Do research on the particular bayonet prior to bidding.  Stay clear of auctions that have private feedback.. There's a reason for that.

While some reproductions you'll notice right away, other's can be difficult to spot, particularly if a devious seller artificially ages it, or alters it to make it appear original.

So far I have encounter reproductions of the following bayonet types.

Japanese Type 30 Variation A.  Tokyo/Koishikawa & Kokura Arsenal marking.
German S98/05 Sawback
German S84/98 III (k98)
U.S 1917 Enfield
British Enfield P1903
British Enfield P1907
British P1879
Austrian Ersatz for Mannlicher 1895
All variations of US Garand Bayonets / Springfield