Stamps are printed using metal plates. The plates are marked with a number on the margins. A plate block consists of the stamps adjacent to that number in pairs, usually four or six stamps.
A plate block is a block of stamps from the edge of the sheet which shows the plate or cylinder from which the stamps were printed.
{| |- | Scott Number 2721, 2724, and 2731 Plate blocks show a value of about 3 dollars. The stamps themselves in used condition are available for about 20 cents. Unused or mint, they can be purchased for less than a dollar. Consult a stamp catalog such as Scott's, for a description on how the stamps differ, as well as identifying the actual date of issue for each of the three types of stamps. |}
Sheets of stamps are printed using plates. Each plate is numbered to insure they know which plate was used so they can correct errors and problems. The number shows up on the edges of the sheet. The post office knows exactly how many sheets were printed on each plate and collectors therefore know what the rarity of the various sheets are. A Plate Block is four or more stamps in a block with the excess paper with the number on it. Most plates print four sheets on a single sheet of paper, so that there are four possible positions for Plate Blocks.
{| |- | Scott Number 1716. You can purchase one of these for about 20 cents used or mint from a dealer. The Plate Block is work a little over a dollar for the four stamps in the corner with the number attached. This minimum value indicates that it has no real value. 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. |}
These stamps are listed in the Scott catalog as # 1771 issued in 1979. The retail catalog value is 40 cents mint and 20 cents used. If you have a full mint sheet or even just a plate-number block you might be able to sell them on e-bay and clear their face value after you pay auction fees - don't hope for much more. If you have used stamps or just a few mint ones, your most likely sales target would be a young collector or someone you can convince to become a collector.
3 dollars with the stamps still on the paper
Unless there is a rare plate block number, they are not going to vary in price much. Most modern era stamps, those since about 1940, are going to be worth face value. Dealers sell full sheets of many common stamps at less then face value and certainly don't pay that much for them. Still, consult a catalog and find out their value. You might be surprised!
The stamp to which you refer was was released on 12-May-1994 as a block of 6 Australian Wildlife design. 3 of the 6 stamps had Koalas on them, the other 3 had Kangaroos. Any one of the stamps if they are uncirculated and in Mint UnHinged (MUH) condition, might get $1.50 each. The entire set as a block of 6 if they uncirculated and in Mint UnHinged (MUH) condition, might get $10.00 AUD.
The letters are called Mintmark's NOT mint stamps. They are used to identify which mint the coin was made at.
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.
Humidity will ruin MNH (Mint Never Hinged) stamps. Stamps should be stored in a moisture free environment.
Scott Number 2982 - 2992 A mint block of 20 of these is valued at $12. So your sheet is probably worth in the neighborhood of $30.