The first flights, naval ships, and older sports related ones you might find some value in. Probably in the 5$-$15 range.
https://en.wikipedia.org/wiki/Cachet
Yes, you can use pretty much any uncanceled official usps issued stamp as postage still.
Most likely very little value and a hard sale. Anything uncanceled that was made for postal use will sell in bulk at 60% or so of face value....
https://trademarks.justia.com/711/29/inbraco-71129326.html
Most toilet fill valves of that type are going to look similar, as they are made to be interchangeable and fit in a variety of toilets. Lots of...
What makes you think Medical?
Bumper Jack
$24.95 (2015)
Nozzle like piece with unusual ending. The tip extends out a small distance, and turns. There is a small hole in the center. Measures 10 3/4...
Those names all look fairly readable compared to some of the paintings/artwork that's been posted. You should be able to work out most of them...
Toilet fill valve I think.
https://www.rubylane.com/item/1061514-0015-3548/Pair-French-Winged-Mercury-Pearl-Brooches
They did do stereoview type cards on postcard stock, but the way the picture is oriented, and the fact that maybe only 1/3 of the card is cut...
$695 (2016) $355.50 (2019)
It's nothing unusual. http://www.metropostcard.com/guiderealphoto.html Scroll down to the bottom, where it says "Borders".
Someone trimmed the picture because they wanted to view it and keep it as a picture and not send it as a postcard. Real Photo postcards could...
Incubator Heater probably. https://www.icollector.com/Incubator-Heater_i26536727
$32.50 (2017)
$149.99 (2016)
Separate names with a comma.
