Hey, @Lee B. I had a chance to look at that list you posted in a little more detail. Here are some things I noticed.
1) First, yes, I've seen your ComicArtAds site. Very cool.
2) Below I highlight the art from the Buyer's Guide Ad that I have scans from. I guess I take comfort in knowing all that great Ross Andru art didn't get snatched up by one buyer. I'm even happier knowing someone did buy it and it all didn't get destroyed.
3) The range of the art is from ASM 133 to ASM 148 and includes GS SM 1 and 4. That puts the date range from March 1974 to June 1975 (using the book on the stands date). Makes sense from a Dec. 1975 Ad.
4) The ad lists 103 pages and I have scans of 22, for a 21% rate. That's lower than the 33% of all Ross's ASM pages that I have scans from. That kind of makes sense, as this is some of his older art on the run.
5) The ad lists pages 17-18 from Issue 136 for $75. I always thought this was a "two story pages on one art board" piece. But based on pricing, I am now convinced this is two story pages on two art boards.
Thanks for sharing this ad, really neat find.