Beside confirming for yourself, the memory part list you made certainly will help others to verify compatible memory for their upgrade.  New or used, these sticks can be had for less than 100 USD on the bay. 

As you said earlier....we shall see.

I got 4 Samsung M393A4K40CB2-CTD on Amazon for similar prices as ebay. I'll take that easy Amazon return in this case. They don't really say new/used, I think they are probably pulls from an unsold system or similar to new, bulk refurbished, something like that. $376 or $94 a module. Free shipping, but $30 tax. Seller has 98% positive in the last 12 months (2573 ratings).  Seller description includes, "We have a very unique supply chain that allows us to incorporate overstocks, liquidations, open box/sealed returns, pulls from brand new systems, and memory in cosmetically new condition from bulk purchases and a vast broker network. We pride ourselves in carrying the largest selection of brands and part numbers in stock."

So, anyone's guess as to the source, actual used/new status, but sold similar to new I'm assuming. Good deal as long as they work! Fingers crossed! I get them on Friday-ish.

Good to hear that you got a good price for those sticks.  It's the same RAM but without the sticker shock.  I hope it will work for you this time.

As for the other issue with forum posting and how it works, Brad can help you to understand better. He's a long time member and a core resident here with a pleasant attitude, always willing to help.  I joined a couple years ago and I only visit randomly a few months out of a year.

@Chino de Oro 

You're also a good forum member and we can always use the help with Precision's.  Often not enough answering over here.

TL;DR

RAM verified with Dell Precision 5820 X-Series with Intel Core i7 9800X:

• Hynix 8GB - HMA81GU6CJR8N-VK,
• Crucial 16GB, 2666MHz - CT16G4DFRA266,
• Crucial 16GB, 3200MHz -  CP2K16G4DFRA32A - relatively easy to buy at the time of writing.
• Likely anything listed here would work, but I've only checked CP2K16G4DFRA32A from above.

More info

I also had problems updating RAM in my workstation. I got some RAM with the exact same parameters as the stick that works (DDR4 16GB, 2666MHz, ...) - no go.

After some more digging I've found this Dell Precision 5820 manual page that explains the issue with 32GB RAM - it only works with the Intel Xeon CPUs, the max for Intel Core systems is 16GB. Yet, just sticking to 16GB, 2666Mhz, 1.2V isn't enough, as mentioned before.

In the end I've found this Crucial page useful - I bought the CP2K16G4DFRA32A set of 2*16GB for my Dell Precision 5820 X-Series with Intel Core i7 9800X and it works (despite the packaging stating that it requires Intel Core gen 12 - 14 CPUs). I think the version without the heat sink (CT16G4DFRA32A) would work as well, but I haven't tried.

In any case Crucial provides a utility that you can download and run locally to scan your system. It produces an html file. When opened the file takes you to their website with components compatible with your system.

I can also confirm that the following RAM works with my workstation:

• Crucial 16GB - CT16G4DFRA266 - this seems to be an older version of CT16G4DFRA32A/CP2K16G4DFRA32A
• Hynix 8GB - HMA81GU6CJR8N-VK - according to this the C after GU6 is just the die generation, so likely any letter there would do.