There were 3 stamps in that series, so it could be Scott number US 1504-6. A full sheet of these stamps has a catalog value of $15.75 or $22.50 depending on which one it is.
The Brookman Catalog is the best catalog to get the current value of sheets of stamps.This stamp is Scott Catalog #1382, issued 1969, 138,055,000 were printed.Brookman catalog value for a sheet is: $18.50
That would be Scott Number US 1755. A full sheet of these stamps has a catalog value of $40.00.
These vary considerably by issue. Unless they have something special about them they are worth face value. It will take some detective work to narrow down the search. Use a catalog online or through your local library to identify the specific item. It will also provide an estimated value.
That would be Scott number US 1745-8. A full sheet of these stamps has a catalog value of $21.00.
Very little £3 or £4, your better of keeping it!
You can get a full page of Boys Town Nebraska stamps on Ebay for 8.00
These are still quite common whether you mean the 20 cent regular issue or the commemorative which was issued soon after he dies. Its retail value in good condition - no missing perforations or thin spots and full gum if unused and lightly canceled if used -- is from 10 to 20 cents retail. Dealer would charge up to twice face for the mint stamps. Wholesale, the value is 50 cents to 75 cents in lots of 100 for the used commemorate and 15 cents per hundred for the 20 cent regular issue. Unused about 65% of face if you have enough unused stamps to make it worth a dealer's time. Full sheets and plate number blocks can often be sold for full face and maybe 20% more.
There is a bit of a conflict here. A stamp cannot be 'Mint' and have a postmark on it, particularly from Canada. There are a number of possibilities here: The postmark is a First Day of Issue and someone asked the postal clerk to stamp them after buying the sheet. Not much value to them as they can't be used for postage and most collectors prefer a full cover with cancellation. You might get a bit more than what a used stamp would bring. The postmarks are actually overprints. Canada has done a number of overprints to change value or commemorate special events. These could have good collectors value to them. Someone re-gummed a bunch of used stamps. Possible, but no real reason for it. It is also possible to use a humidor to remove stamps from paper and 'reclaim' the gum on the back. Most collectors would classify these as used stamps.
Scott # 1095 Full sheet with First Day cancellation. Individually, the stamps can be purchased used for .15 each. As a sheet with FDC cancellation, the value would be difficult to determine. It is an unusual item and could get 10 or 20 dollars from a dealer.
{| |- | Scott Number 2967. You can purchase one of these for 20 cents used and about 85 cents mint from a dealer. The full sheet of 20 stamps goes for about $25. Dealers typically will purchase stamps at around 25% of the catalog value. Consult a stamp catalog such as Scott's, for a description on how stamps are rated and graded. |}
Someone's used passport will have used stamps. A stamp album will have many used stamps. And most scrapbooks have many stamps in them.