Should I sell my old RAM or keep it as a spare?

Sell it if you can get $30 or more. RAM almost never fails, and keeping a spare 16GB kit is usually overkill—you’ll likely never need it, and it’ll be obsolete by then. Check eBay or r/hardwareswap; DDR4-3200 or faster in decent shape can fetch $30–50. Keep it only if you have a second computer that could use the upgrade or plan to build a test bench.