First, the Asus Sabertooth X79 specifications rate the board's memory speed as 1866 maximum:
Asus Sabertooth X79
Why that is, I don't know, but I've noticed that Sabertooth boards tend to have lower maximum memory speed specs.
If you check the board's memory QVL: Motherboards - SABERTOOTH X79 - ASUS
... you'll see that some memory up to 2400 is included. But notice on the right of the column headings, the number of DIMMs that may be used with a particular model of memory. As the memory speed increases, fewer models can be used with 8 or even 6 DIMMs. Even at 1866, most models listed can be used with only four DIMMs.
It sounds like the memory you got at auction were all single or maybe dual DIMM packages, is that right? If so, they are not a quad channel matched set, which also makes it harder for them to work together at the maximum supported speed. Frankly, you're lucky they work as well as they do, given random single or pairs of DIMMs.
What you can try with the memory you have, to get it all recognized, is lower the speed. As a test, manually set the memory speed to 1600 or 1333 with all the memory timings and voltage on Auto to make it easy for it to work. If it all is recognized in Windows, great, you can then try to increase the speed one step at a time (1333 to 1600, 1600 to 1800/1866, etc) until you get missing memory again. Whatever that speed is, the max speed you can use is one step below it. After that you can set the memory timings from Auto to the values shown in the CPU-Z SPD tab for whatever speed you can use. It seems like 2400 with 8 DIMMs is to much to all work together fine.
Also, when using all the memory slots, the CPU's memory controller will likely need a boost in voltage. This is not the same as the memory voltage, which may also need to be increased if it is currently at 1.5V. I'm not sure what your BIOS calls that setting, could be System Agent or something else. Check your manual or the Asus forum for your board for more information.