I know it’s illegal to build an 80 percent lower and sell it or a rifle built on one,  but does the owner/builder have to do the machine work themselves to remain legal?  In other words, can I buy the lower, then have someone who has a jig finish it, then finish the build?  Also, can 80 percent lowers be completed and stored for the future since they have no serial number.  DO NOT want to run afoul of ATF

If someone else completes the lower for you he must be a licensed manufacturer, mark it with all the required markings, and transfer it to you on a 4473.

You can finish out a 80% lower and put it under your bed for as long as you want.

And you can sell it later. You just can’t built it to sell.

Yes.  No.  Yes.

You can't go around selling completed firearms you made yourself if you're making them for sale - if you mill out an 80% lower, and then your buddy wants to buy the rifle/pistol/shotgun from you down the road, it's legal to sell.

You can sell a firearm that you have built, but I don't recommend building on Saturday and selling on Sunday, that would be considered building with intent to sell, which you have to be a licensed manufacture to do.  If you can find an FFL 7/SOT willing to do the machine work, then no problem, but he will have to mark the lower with his information and record it in his records as a gun he manufactured, so it will have a serial number on it that he chooses.

You have to do the milling work yourself if you don't want to find a manufacturing FFL that will do it for you, and these days, even borrowing tools is a sticky wicket, they say you have to complete with your tools.

As far as completing and then storing, no problem, I have a half dozen 80% on the shelf and around the same amount that I have completed that might be used for future builds, if I decide to sell the completed lowers, I will have to put my information on them and record them as part of my yearly total that I manufactured as I do hold a FFL 7/SOT, but any non-licensed person can complete and store as many lowers as they want to and you can sell them at a later date if you so desire, just make sure you are not doing them with the intent to sell them.

At the price of stripped lowers these days, if you don't have the tools, I would just suggest buying a few of the stripped lowers then build them at your leisure.