The biggest issue with Dell and cpu upgrades is Dell's bios. It's not an aftermarket, standard bios but one written by Award or Phoenix specifically for Dell. There's very very few Dell pc's that come with a K squ cpu. The only guaranteed cpus that do swap are the ones in that particular model family on that motherboard. So chances are you could use anything from an i5-6400 to an i7-6700 or the i5-7400 to i7-7700.
Cpu sales for the most part are bottoming out. Many ppl went to teleworking at home or bribed their now homebound kids with a new pc during Covid, and the stimulus checks, but that period is over and ppl have decent pc's with little incentive to upgrade. Especially with today's absurd pricing.
So the used market is drying up to some extent and prices won't bottom out, demand replacing supply.
That means newish components that are aged will be rare and the old stuff more expensive than it should warrant.