It would help to scan the stamp at 1200 DPI or higher. The image presented appears to have been enlarged after scanning. At 1200 DPI it is possible to see differences in width to a tolerance of +/- 1/1000 of an inch. From the image it appears that stamp is slightly narrower at the bottom than the top. The difference is likely on the order of 0.003 inch. The edges should be parallel to within 0.001 inch. Also, the perforations appear to have been added. It should be noted that earlier fakes were not exactly perf 12-66. The test is to match the perforations with a perf 12-66 sheet stamp. Even if the edges did check out, it is possible that the stamp would be from a vertical imperf coil (347V).
In any case, buying a perf 12 coil without a certificate is an almost sure recipe for disappointment. One well known seller has sold amazing quantities of coils and Scott 519 without certificates. Check the seller sold listings on eBay
in Stamps > United States > 1900-1940 Unused sorted descending by price and you will see active listings: http://www.ebay.com/sch/1901-40-Unu...045573.m1684
Then select sold listings to see the "good stuff".