I would be careful about selling them for little money before some research. 7308's were made and/or sometimes rebranded by Amperex, Siemens, Sylvania, Raytheon, and probably others as well. If you can post a close up of the tube, I am sure they can be indentified. Remember, these are decades-old and Amperex 7308's (a GREAT tube) did not cost $300 back then. Gold Aero was known to select premium samples of Amperex tubes for their own brand. You could very well have some very good and very valuable tubes.
This image of a Gold Aero 7308 sure looks like an Amperex to me:
https://www.google.com/search?q=gold+aero+7308&hl=en&client=safari&tbm=isch&tbo=u&source=univ&sa=X&ei=PMtVUYieFNWo4AOm04HIBA&ved=0CDUQsAQ&biw=1024&bih=672#biv=i%7C1%3Bd%7CQopwsV8Q7RGBOM%3A
However, this one does not:
https://www.google.com/search?q=gold+aero+7308&hl=en&client=safari&tbm=isch&tbo=u&source=univ&sa=X&ei=PMtVUYieFNWo4AOm04HIBA&ved=0CDUQsAQ&biw=1024&bih=672#biv=i%7C2%3Bd%7C6Ls7VaBf8qGAEM%3A
Good luck.
This image of a Gold Aero 7308 sure looks like an Amperex to me:
https://www.google.com/search?q=gold+aero+7308&hl=en&client=safari&tbm=isch&tbo=u&source=univ&sa=X&ei=PMtVUYieFNWo4AOm04HIBA&ved=0CDUQsAQ&biw=1024&bih=672#biv=i%7C1%3Bd%7CQopwsV8Q7RGBOM%3A
However, this one does not:
https://www.google.com/search?q=gold+aero+7308&hl=en&client=safari&tbm=isch&tbo=u&source=univ&sa=X&ei=PMtVUYieFNWo4AOm04HIBA&ved=0CDUQsAQ&biw=1024&bih=672#biv=i%7C2%3Bd%7C6Ls7VaBf8qGAEM%3A
Good luck.