From the Corsair product list:
https://www.corsair.com/ww/en/Categ...z-C16-Memory-Kit---Black/p/CMK16GX4M2D3000C16
If you have two sticks and you put them in two different channels on your motherboard, you will be operating in dual channel mode.
The only issues is that sometimes, unmatched sticks, even with the same product and lot number are incompatible or will run poorly.
If your seller will take the ram back, and you trust him to do so, then try them.
You can verify dual channel operation if you run cpu-Z and look at the memory tab.
I do not want to dissuade you from what the seller is offering.
It is almost certain to work. Any honest seller should take the ram back if he said it would work and it fails,
The seller is mistaken in asserting that the ram is not sold in kits.
After installation, test the ram.
Run memtest86.
It boots from a usb stick and does not use windows.
You can download the free edition here:
https://www.memtest86.com/download.htm
If you can run a full pass with NO errors, your ram should be ok.