Fakes and forgeries
Whenever there are objects of value, there will be fakes. This is the case with rare pens as well, although outright fakes are still relatively uncommon and generally fairly obvious to an expert eye. Of greater concern to the typical collector are "enhanced" pieces, mismatches, and excessively restored specimens.
At the higher end of the market, we have recently seen a number of Waterman and Montblanc pens enhanced with Toledo-work overlays taken from other pens of less prestigious brands. Genuine Toledo overlays do exist, of course, but the fit of the transplanted overlays is generally quite sloppy. We have also seen several fake Waterman silver overlay eyedroppers in circulation, including a full-overlay taper-cap, an 8-size straight-cap with hand-hammered decoration, and a "404" with floral high relief work. These bore fake Waterman marks on the posting ends; all originally came from South America.
Other problematic pens you are likely to run across include: any Waterman lever-filler filigree over red hard rubber (genuine examples are nearly nonexistent, while assembled pieces are common); Parker 51s with hand-hammered silver caps (again, originals simply do not appear on the market, while lots of newly-made ones are around); Parker 51 "demonstrators" with recently-made transparent parts (no imprints, interior finish lacking, especially at the ends); Parker 51s in wildly-colored plastics (flooding the market via Argentina); Duofold Senior Lucky Curve Deluxes with the wide cap band that covers the cap lip (almost all these cap bands are newly-made, and cover up a damaged or restored cap lip); and indeed, almost any very large hard rubber pen with a nonstandard cap band. In addition, one must be very careful about buying any high-value Montblanc or Pelikan with a metal overlay, since fakes have become common and have even been illustrated in reference books and offered at reputable auction houses. Some are beautifully engine turned, and may bear "factory" imprints (generally engraved with a pantograph, rather than stamped). Another unfortunate development is the manufacture of unmarked reproduction Parker sword clips. These do appear to be cast rather than struck, however, so can be detected upon close inspection.
Common mismatches include: silver overlay pens with nickel clips or levers; Waterman 100 Year pens with bands on both cap and barrel (mismatched: bands were on one or the other, not both); Dorics and Coronets with Skyline nibs (correct on some late examples, but much less desired by collectors); Duofolds with arrow nibs and finned feeds (the same situation); and red hard rubber pens with replacement caps in celluloid.
For the latest on pen market fakery, take a look at our blog posts at Vintage Pen News.