You can buy most guns from private sellers. I say most because true full auto guns, amongst other restricted weapons you have to have federal level government license to own. Where it is an AR-15, you should be able to buy it no problems from your friend. But i would look up utah law on buying privately owned guns and what Utah's law says about bump stocks just to be safe.
Yes- bump stocks are legal to own, no longer legal to manufacture new ones.
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