The Five Cent Imperforate Stamp and Fakes
I am frequently amazed at the prices realized for small margined 5-cent
Lincoln imperforate stamps on eBay (and probably stamp bourses and other
auction venues). The stamp known by its Scott Catalog number 315 is frequently
traded on eBay at what appears to be bargain prices (click
here for current eBay auctions of 315). 
The
2001 Scott price for an unused example (in very fine condition) is $290
and a used version is much more uncommon, commanding a price tag of $475.
In fact, legitimate (read as certified) copies of the used stamp are quite
scarce, the price in the Scott catalog reflects this by italicizing the
price. Comparing the imperf to its perforated counterpart (Scott 304),
an unused (previously hinged) stamp catalogs at $55 and used a mere $1.50.
Serious philatelists collect the imperforated stamp either in imperforate
pairs or a single side/corner margined copy, thus guaranteeing authenticity.
The perforate version does not exist in either of these states.
Unfortunately, over the past century forged copies of the imperforate are plenty. Some clearly created to swindle the stamp collector others contrived to fill the void in a stamp album (and not intended for resale). Either way, the fakes come to market.
I decided to dig through my stamps and covers to find a large margined
copy of the perforated 5¢ stamp ready to digitally fake. 
The
digital fake could then be used to compare with other stamps encountered
for sale online. If the margins of the stamp in question are smaller, it
could have been faked. 
Ultimately
I spotted a beauty in my collection, a huge margined stamp on cover to
Warsaw, Poland. The stamp is an upper right corner margin copy with rather
large margins around the perforated sides.
Digitally cropped stamp.
Click to enlarge.
Scanning, digitally cropping the perforations, and adding a black background, I now have a superb (in my opinion) digital Scott 315. Is this stamp worth 475 bucks? Or even ten to twenty times a used perforated stamp (eBay rates)? I personally don't think so. It ultimately depends on what you are looking for genuine or space filler.