I wouldn’t advise a used toaster oven for powder coating. New ones are cheap, and I was told there are too many used toaster ovens that don’t regulate temperature properly (it’s a common failure mode).

I’ve powder coated hard-cast lead bullets in a (new) toaster oven, and it worked just fine. I suppose if you’re powder coating something with a much higher melting point than HCL, unregulated heat might not be as bad if you watch it closely. But you can still ruin the powdercoat finish with high heat.