There are a couple of issues at work here that decrease the value. First of course is the repaired tear which is pretty visible. The other is the cut off perforations on the left side, but it's not just cut off perforations, because, in view of the corner letters, this is a so-called "wing margin" stamp. A note in the Stanley Gibbons catalogue explains what these are, an extra wide margin on one side of certain of the surface-printed stamps from 1862-1867. In the case of the 9 pence stamp with emblem watermark, having the letter D,E,H or I in the lower right corner, it was on the left side and about 5 mm wider than the normal margin, like the example below. A few years ago I bought an unused copy of SG 121, the very scarce two shilling brown stamp from the 1867-1880 series, with the wing margin cut off and a Specimen overprint. Scott doesn't list these with the overprint, but SG does. It lists that stamp at 3800 pounds, but largely because of the straight edge I was able to buy it from a UK dealer for about 300 pounds, less than a tenth of catalogue price. This explains the offer you were given.