Uh, if it's barrel, barrel nut, front sight tower included, the flash suppressor didn't have to come off to pull the barrel. The question is: If it's a good barrel, why is the original owner getting rid of it? Second question: Do you trust him?
Barrel changes on an AR don't require machine work, but do require specialized tools to do right. You'll need at minimum: barrel nut wrench, blocks to hold either the barrel or the upper receiver,a torque wrench, a smidgen of grease for the barrel nut/barrel shoulder interface and headspace gauges. You'll also need to be sure the goose neck on the carrier enters the gas tube without hitting the tube. Probably cheaper to pay someone who already has the tools.
Yeah, you can throw the barrrel in there and trust the gun gods on headspace, but you're the one with your face by the receiver while you're firing it.