I was in a similar situation when I had an A320 mobo with only 2 RAM slots and 2x8GB sticks. I wanted to upgrade to 32GB. Fortunately I had a friend who was building a new PC with only 16GB RAM so I told him to buy 2x16GB instead and I'll pay the difference/agreed amount and would trade my 2x8GB with him.
Since then I upgraded to B450 with 4 DIMM slots due to an unfortunate BIOS corruption on my A320 and I have an upgrade path to 64GB now.
My suggestion is to sell the sticks to someone instead of trading as it'll be hard to convince people they need 32GB RAM for a smooth PC experience. People still think 16GB is fine (it might be for them) but once almost all pagefile I/O is gone the PC is very smooth.
Windows does use more RAM even if you aren't using it. If it's not available this "Standby" portion of memory is dumped in the pagefile and reloaded when programs are loaded again.
So, sell it and as soon as you sell one pair you can at least buy 2x16GB sticks and then sell the other pair.