As far as sales go, check this German website: https://popkultur.de/die-20-erfolgreichsten-bands-der-70er-jahre/
Here are the Top 10 according to them:
1. The Eagles
2. Led Zeppelin
3. Pink Floyd
4. Aerosmith
5. Fleetwood Mac
6. CCR
7. Supertramp
8. Kiss
9. Lynyrd Skynyrd
10. Queen
Does that include total sales for just their 70's work or for all of it? Seems like half of CCR's output was from the 60's.
I suspect the Eagles sit at number 1 because their first greatest hits is like top 5 now in all-time sales.
As much as I like Supertramp, I am surprised to see them in the top 10 there.
I can tell you what it's NOT: US Sales for just the 70's albums. Because Rumours alone outsold the entire Aerosmith 70's catalogue (in other words, the total US sales of Aerosmith -> Right In The Nuts is about 20 million, and Rumours itself is two times Diamond, or 20 million.)
And again, using US sales for just the 70's albums, Led Zeppelin is just shy of 90 million sold (including FOUR Diamond albums). Eagles (it's "Eagles", not "THE Eagles") are at about 60, with the one massive Greatest Hits record (3 times Diamond, or 38 million).
(EDIT: By the way, not being a xenophobe; I understand it's a German site, and so not likely to rely JUST on U.S. sales; however, "gold" and "platinum" records are counted differently in each country. In the U.S., "gold" is 500,000 sales, and "platinum" is 1,000,000. For Great Britain, if memory serves, Gold is 400,000, and Platinum is 600,000. Rather than do the math for every country, I used U.S. sales as a proxy for global sales. It probably skews FOR Eagles and Aerosmith and AGAINST Zeppelin, Floyd and the Mac, but it's still an approximation.)