Note that a lot of Costco’s will sell you a roll of 100 0.37 stamps for $36.50, or .75. I wonder if any Postal employees have a (kinda 
slow, but low-risk) retirement plan of buying rolls there and selling them at work?
I’m sure Costco makes it up in volume.
Re: using a rare stamp for ordinary mail
It wasn’t intentional, but there was a case some years back.
The Postal Service was going to issue a set of commemoratives for black cowboys. After they’d been printed and distributed to various post offices, but before the issue date, the descendents/family of one of the cowboys notified the Postal Service that they’d made a mistake on his stamp. They’d gotten the picture of the guy’s brother or something. So the Postal Service recalls the sheets of stamps and sets about to fix the error.
But it turns out that one post office (in Bend OR, IIRC) had jumped the gun and sold two sheets of the stamps. One of the sheets was still owned by the purchaser who was now a much richer person. The other purchaser could not find the stamp. It had apparently been used for ordinary mail, perhaps for a utility bill. Much wailing, gnashing of teeth, etc.
My bank does it for free…
As for stamps, they’re sold everywhere in Canada at cost. I believe Canada Post has worked out some sort of arrangement with vendors to that effect. (and it aggrieves me muchly that I have to go to the post office to buy stamps now that I live in Manhattan.)
There’s more to the story than that. The cowboy in question was Bill Pickett. When the stamp was issued, it was discovered that they had inadvertantly used a picture of his brother, Ben. They tried to recall 5 million of the panes, but 183 of them had already been sold. So, in a decision that infuriated a lot of stamp collectors, the USPS decided to sell 150,000 of the panes of stamps through a lottery. (It must not have been a big lottery. I won one pane of the stamps while my father won two!) This had both the good result of allowing a lot of collectors to own an “error” that they might never had been able to afford, but the downside was that it dramatically reduced the value of the original error.
I should also add that since this happened, and since most collectors were loathe to use an “error” stamp as actual postage, this is one of the rare occassions where a cancelled stamp actually has more value than an unused one.
Sidenote for U.K. Dopers. Here, is seems, convenience shops and so on may sell postage stamps for over the “normal” price, but IF they do, they ought to be paying VAT on the profit made. SO, the Royal Mail is quite happy to be informed when shops are selling stamps at over the face price, and they seem to think that a little letter explaining about the VAT has a tendency to persuade shops that they might as well sell at the face price, (not, perhaps, wishing to attract any attention to their VAT returns).
Yes, the extra penny doesn’t matter much , but the shop I have in mind is rather a sneaky shop, failing to display proper prices and so on on a regular basis, so I did feel irritated enough to check out their claim that “the price has gone up”.
Heheh - AND the Royal Mail then sends me, the one who complained, a nice little book of free stamps.
Despite the extra trouble gone to by the shopkeeper to order and sell stamps, I am pretty sure they benefit anyway - if popping into a “convenience” store to get some stamps, I am very likly also to buy stuff like milk, newspaper, cigs, chocolate… - things I would otherwise have bought elsewhere.
Many sitcoms eventually have a similar episode with coins. Archie Bunker for example, comes across a very rare dime. Excited, he calls Edith, only to realize that he has just used the rare dime to make the phone call. Oh, that Archie!
Back to the OP, stores can also do the opposite. There’s a chain of grocery stores around here that sells stamps at a loss (Say .30) just to get you in the store. They’re banking on the fact that if you go in to buy a book of stamps, you’ll also buy something else, making up for the loss.