My largest single-stamp purchase at Chicagopex was an upgrade for my R31c, the 6c Proprietary. These are almost always found faulty and poorly centered. They were issued very late (the first delivery was April 18, 1871) to address a need on proprietary medicine products. The stamp was specifically requested by Charles Osgood because the government was requiring a 6-cent tax to be paid on a $1.50 purchase, yet was not providing a 6-cent stamp to do so (the 6-cent Inland Exchange could not be used; you had to use proprietary stamps on proprietary transactions, not any of the other types). There were not many issued, and were frequently used on boxes, packets, and other ephemera, hence their being frequently found damaged.
The Scott catalog states:
"Nearly all examples of No. R31 are faulty and poorly centered. The catalogue value is for a fine centered stamp with minor faults which do not detract from its appearance."
There's two important things to note with that statement that differ from how other stamps are priced: (1) the SCV is for a stamp with F centering, not VF, and (2) the SCV is for a stamp with minor faults, not sound.
First here's my old one, with PF cert. It's a bit of an ugly duckling, with a Charles Osgood oval handstamp cancel. With that right side, this one was discounted quite a bit when I got it, hence my ability to afford it at the time.

After a while, even though the spot was filled, I decided that I wanted something nicer. I recalled a dealer that had two of them that I looked at last year at Chicagopex; he was quite firm on the price at the time. I knew he'd be there again this year, so I figured I'd check and see if he had one or both a year later. He still had both. I examined the two, and one was clearly nicer in my opinion, and also had a better cancel. I made him a cash offer, explaining that it was based on the fact that he hadn't sold either example in at least a year.
He countered, but I held firm, and he finally capitulated. I ended up with the stamp at a price I'm very happy with. As it turns out, I came back past his booth on Saturday, and he said that he had sold the other one as well. Both R31c sold at the same show, after both going unsold for at least a dozen shows.
I'm glad I made him an early stop on Friday, as I might have lost the nicer of the two.
This one is clearly an upgrade. Much nicer condition and scarcer cancel. The 1998 APEX cert says that it has a "thin spot" but we dipped it and could not find it. If it's there it's tiny. The stencil cancel is from "A.M.F Gianelli" a seller of Italian bitters.
I know I've done something right when both Eric Jackson and Richard Friedberg tell me I bought a stamp way too cheap.

