New answers tagged vintage
Those would be Waffle Bottoms, which seem to have been mostly phased out by 1969, except for the 4x8 plate that went until 1971. Having not read any deep history on old LEGO designs, I'm going with that was just what they went with to start, then realized that the tubes were much better.
BrickLink or eBay. You could get significantly more if you go through the lot and separate out full sets then sell them on BrickLink. But if you don't have the time at least take a lot of really good pictures - in this case an eBay auction would be the better choice.
Brick Link is a site for buying and selling Lego. Although it focuses on Lego fans, who will take the time to enter each piece separately, they do have the option to sell 'unsorted lots', so perhaps that's an option for you.
ATC is the Asahi Toy Company from Japan. Here are the pages from their 1971 catalog showing some of their construction sets:
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