> Straw purchases are only when buying for someone that can’t legally buy a gun.
Not quite https://en.wikipedia.org/wiki/Abramski_v._United_States
> There should be no issues with buying a gun and then deciding to sell it to someone eligible after quickly deciding you don’t want it anymore, no?
Yeah it's confusing because you can buy a gun with the intent to gift it to someone. In your example, it would come down to proving that you lied when filling out the 4473.
> What are the laws about carrying your spouse’s gun?
Depends on the state. In my home state of Arizona, none. I can carry my gun, my wife's gun, or a friend's gun not even a carry permit required. Aside from prohibited possessors (felons, etc.), I can loan or transfer to anyone who is of age a gun. Moreover since Arizona has no registration and is a community property state, "ownership" is no different than any other piece of property and considered both of ours.
In my birth state of California, you can only carry a gun that is on your permit, and as far as I know, you can get a gun on a permit that has been registered to you (either through DROS or self-register when moving to the state---note that possession of an unregistered gun is not a crime).
So unless the state prohibits the transfer without a background check through a dealer, you can probably carry your spouse's or friend's gun