Yes, definitely some worries. What if the used components are dead? If everything works, how long before something breaks? My worry would be obtaining parts from those who overclock without adequate cooling.

Memory, nvme, case, cpu heat sink and fan, power supply keyboard and mouse I purchased new.

The motherboard board was a little dusty with some animal hair, it cleaned up ok. It also smells heavily of e-cig lol. Definitely some vape residue there. I would say dust and general cleanliness is an issue, but didn’t bother me too much. If you’re buying a fully assembled pc used, I imagine it could be very dusty and gross depending on the previous owner and the environment it was in. Good luck in your search.