They generally have less processing power for a cheaper price because the company had to fit hardware into a machine smaller then the ps5 and they will have to cut corners, generally with processing power, also he already bought a ps5, and probably opened and played with it, lowering its value, if he sells it, then he won’t get back 100% of the money he bought it for.

Edit: they aren’t more power efficient, power efficiency is a metric of how much processing speed you can get for the amount of electricity you are using, slim consoles use less power sure, but only because they have lower processing speed.