Over the last two years I've been collecting a wide variety of the line-engraved issues of British Queen Victoria stamps, mostly bought on eBay. On several occasions I've ended up with forgeries- some sold intentionally, some by accident, but it has become apparent that there are some so-called "dealers" out there who have no scruples when it comes to selling forged stamps. I've come across the following on eBay:
Penny Blacks which have been copied onto thick paper using a laser printer, damaged, and stained with tea to look old;
Penny Blacks which have been printed on a dot-matrix printer (unbelievably!);
Twopenny Blues with white lines, which have had the white lines accurately coloured blue to look like the unlined versions;
Penny Blacks and Twopenny Blues which look genuine, but do not fit any of the known patterns of plate numbers (these are the most worrying, as they are very cleverly done)
And I'm sure there are various other types out there in the marketplace.
My advice to anyone aiming to bid a lot of money for penny blacks or tuppeny blues- try and plate them out first, before bidding, and never, never bid for a stamp where the photograph looks blurred, or the wording is ambiguous. Don't take the dealer's word for it being a certain plate number, check against references, Nissen's photos being one example that I use. Don't rely on the dealer's feedback score either- two of the forgeries I've mistakenly bought were from dealers with 100% positive feedback.
Be aware that there a lot of forgeries out there! Caveat emptor.