Value of Stamps?

Value of Stamps?
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Greetings,

I'm a newbie, have read the FAQ, and have two questions for you experts
about the value of some stamps I have.

I'm asking here because years ago I was badly cheated by a stamp buyer
who paid me $150 for a sheet of 100 misprinted 50-cent stamps - and then
he sold them for $1,500 (!) to a collector.

Are the following stamps worth just the 29-cent face value or more?

All are plate blocks (?) in mint condition. All are 29-cent stamps:

1x "Country & Western" - 20 stamps/plate

3x "Popular Singers" - 20 stamps/plate

2x "Jazz Singers/Blues Singers" - 35 stamps/plate

1x "Elvis" - 40 stamps/plate. The left upper stamp has a red
Post Office stamp on the left side.

1x "1944: Road to Victory" - 20 stamps/plate plus two large maps
(3 by 8 inches) in the center.

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I also have one "U.S.INTER.REV. - PROPRIETARY" - four cents. It has two
small white spots (1 mm). This stamp can be seen here (124K .jpg):

http://aman.members.sonic.net/stamp.jpg

How much would that stamp be worth?

Thank you for any help you can offer.

Rey Aman
Santa Rosa, Calif., USA