I intend this thread for education with regard to the fact that for Classic US stamps, the term 'Unused' does not necessarily mean that the stamp never went through the mail. It often means 'Uncancelled'. There is a significant distinction which all collectors should be aware of.
This is an issue I have with terminology, whereby the literal interpretation of the word 'Unused' implies that a stamp was not postally used. I would stipulate that for stamps with no gum, that are described today, as Unused, a better term would be 'Uncancelled'.
I was reminded of this after seeing a US Scott #5A lot in today's Siegel sale which I thought would be very useful to make this point really clear in people's minds. To be perfectly clear and fair - the Siegel description was up-front and provided full disclosure here, although it really didn't need to.
The Scott US specialized catalog makes a statement that prior to a certain date/issue, that prices for Unused stamps assume no-gum is present. This is because, going back to the mid 19th century, many if not most stamps that were truly unused have had their gum soaked off, often to help preserve the stamp. Gum can accelerate deterioration of a stamp in some cases. Stamps that still have original gum from this time period carry a significant value premium. I'm avoiding the obvious topic of fraudulent regumming here, as I want to focus on Unused no-gum stamps, and use this 1c 1851-issue stamp as an example.
The following stamp was Lot 23 in Siegel sale 1187 today.
It was described as: "1c Blue, Ty. Ib (5A). Position 5R1E, unused (no gum), large margins to in, rich color, small toned spot at bottom".

Image courtesy of Siegel Auction Galleries
The Ty Ib 1c 1851 stamp, Scott #5A is a really rare item in Unused state. This Type only occurred on 6 positions from Plate 1-Early. That plate was in use for about 1 year, from July 1, 1851 until mid 1852. So this is a pretty nice stamp, even if it is slightly cut in at left and bottom.
For those people who collect more modern stamps, the condition of 'Unused' typically carries with it the connotation that the stamp really was never used on a letter. Such is not the case for early US stamps such as this. Since the gum is missing from many early US stamps that were indeed once Mint, and never used on a letter, the term 'Unused', for these earlier stamps, was relaxed to mean 'Uncancelled'. This creates a scenario where one cannot usually tell if a stamp went through the mail on a letter, but was not cancelled, from a stamp that never went through the mail, but has no gum. The convention is that they are treated equally in terms of value and are both called Unused.
Getting back to the #5A above, the description from Siegel, is that it is Unused with no-gum. Remember, Unused means Uncancelled. In their description of this lot, Siegel points out (to their credit) that this stamp was once part of a strip of three: 5-6-7R1E - Ty Ib-Ib-I. They note that 6R1E and 7R1E were both cancelled, but the 5R1E (this stamp) was uncancelled. The strip has been broken up into three single stamps.
The following is a photo, courtesy of the Philatelic Foundation, of the original strip from its PFC:

You can clearly see the cancel on the right two stamps, while the left stamp, our #5A appears Uncancelled.
Here is what the 6R1E looks like now:

Image courtesy of Siegel Auction Galleries
Here is what the 7R1E looks like now:

Image courtesy of Siegel Auction Galleries
The original strip of three stamps was clearly postally used, and went through the mail at some point. The single letter rate in this time frame was 3c, so it probably paid a single letter rate. The strip was soaked off of its cover and broken into three singles. Now is where it gets interesting. The left stamp, 5R1E, is and can be classified as Unused, while the other two stamps (6, 7R1E) are classified as Used. We know, of course, that the 5R1E went through the mail. That has been factually demonstrated. Our system of classifying early US stamps, however, just states that if a stamp is Uncancelled, then it qualifies as Unused. If it has Original Gum on top of that, then it carries, often, a large premium.
I thought that this was an interesting way to illustrate what Unused means, while looking at some pretty cool stamps in the process.